So how were slaves and free people told apart?

No; in states which allowed slavery, the law was always that a child born to a slave mother was enslaved, regardless of facial features or skin hue, unless and until the master freed him/her.

The censuses of 1850 and 1860 required enumerators to subdivide African Americans (slave or free) into “black” or “mulatto”. This is obviously imprecise and we can’t be sure how much attention any enumerator gave to this classification.

Nevertheless, for what it is worth, the census of 1850 classified 411,000 (10.4%) out of 3.95 million slaves as “mulatto”, and 177,000 (36.2%) out of 488,000 free blacks as mulatto.

Assuming the enumerators took any care at all in their work, we may safely conclude that the 89.6% of slaves classed as “black” didn’t look like Britney Spears and couldn’t pass for white. Given the choice between “mulatto” and “black”, an enumerator classed them as black.

Of the 411,000 enslaved “mulattoes”, we have no way of knowing how they looked. No doubt they covered a wide spectrum from “slightly lighter-skinned black” to “looks like white but I’m not sure”.

Note, however, the larger percentage of free mulattoes. There was a tendency for lighter-skinned people to become free, for a variety of reasons: masters were more likely to manumit children who “looked like them” (mostly white), white-looking slaves found it easier to run away and pass, and masters didn’t always want white-looking slaves because they wanted dark-skinned slaves to believe that race and slavery were synonymous.

Given which, if I had to guess, I doubt that more than 10% of enslaved “mulattoes” (1% of all slaves) could have passed for white, even on a good day in the right light. Maybe 20% and 2% at the outside.

Of course 1% of 4 million is still 40,000 people.

Lots of mixed race slaves were freed because their owner was also their father, or their uncle, or their grandfather, or their half brother. That’s why they “looked like” them. The parentage of mixed race slave children might have been kept out of the record books, but was well known and a major subject of gossip. It was one of those things that no one talked about in public but everyone knew.

And of course, the likelyhood is if you owned a slave that looked like Britney Spears she was either your daughter or your concubine. Mixed race slaves were priveleged not so much because they were white looking, but because they were literally relatives.

Here’s what people are missing, because you’re all both right and wrong: the racial component of slavery was deliberately created by slaveowners after the Revolution.

Post-Revolution, many slaveowners actually freed their slaves, creating a substantial free black population. Slavery wasn’t black then, not entirely, and black and white slaves (or indentured servants, as the line between them was very dubious) weren’t treated much different.

However, those slaveowners that didn’t free theirs sought to create barrier in society. They wanted to keep the poor and middle-class white majority from sympathizing with slaves and ending the institution. Thus, they freed their slaves -their white slaves. And they developed the idea that blacks were slaves and slaves were blacks so effectively that two generations later their descendants were willing to fight, kill, and die to defend it.

However, this is somethign which developed - deliberately created - in American society. It didn’t start that way and wasn’t always that way, and even when it was, not every slave had predominantly black ancestry. However, slaves which got a little too white could more easily pass into the white world, and were probably more likely to obtain freedom. That said, plenty of slaves were the sons and daughters of their masters, and were sold or simply kept at labor regardless.

From the first picture, “Charley” is blonde, as well as I can tell from a 150 year old sepia tone

Of course, these pictures were propaganda printed by abolitionists, intended to inflame the emotions by showing enslaved white people. Of course the vast majority of slaves were plain old black and the minority that were mixed race were still obviously mixed race. But even if the percentage of passable mixed race slaves was low, that’s still thousands of Lena Hornes and Wentworth Millers.

Thousands of Maya Rowell’s, thousands of Anna Becker’s, thousands of McDonald sisters.

This wasn’t rare. Not then. Not now.

How slavery came to be associated with race in English speaking North America, late 1600’s:

http://backintyme.com/essays/item/12

Well, that’s the point-- you can’t tell. So far, no one has presented any definitive evidence of a “Brittany Spears” phenotype slave. No need to go overboard here. There were thousands and thousands of slaves whose ancestry was majority European. There is no need to exaggerate to BS, pun intended.

Kidnapping and selling free blacks back into slavery was a major illegal business, and one that some slavecatchers were notorious for being involved in. John Brown helped start a community for freed slaves in New York that had 24/7 security patrols by dogs and men with guns to ward off this very thing, as did some free black communities in Ohio. Once captured it was next to impossible for a captive black to prove he or she was free, starting with the fact that few were inclined to even listen.

In Rome tattooing was very common. If you watch the HBO series you’ll notice that Pompey and Atia both have slaves with their (the owners) names tattooed. Some slaves also wore shackles on their neck and wrists- not chains, just shackles- and medallions identifying them as the property of ______ and guaranteeing death of anybody who broke this chain.

In Virginia there were more than a few slaves who were legally white; once you could prove that more than 3/4 of your ancestry was white (which was not easy to do obviously- not a lot of whites acknowledged mulatto children- but it was done at least in a few cases) you were legally white, BUT it did not change your status as a slave. Sally Hemings had two nieces who were probably 7/8 white and both were white in appearance, and both in fact got permission to marry white men. In both cases the white men bought the bride, freeing them later (mainly because they had to pay the money used).

The Irish men who were sent to the Caribbean (and a few wound up in Virginia and Massachusetts) by Cromwell were differentiated less by their looks than by their accents and in some cases their language (for not all of the Irish spoke English). Most didn’t live long enough to run away- the tropic heat and diseases made short work of them. The masters undertook breeding progams of mating Irish women to African men to basically make the babies hardier, a practice that was outlawed in some places by anti-miscegenation legislation. (Jamestown had such laws earlier, first for Indians and then for Africans, but since the white population of the islands was mostly male anyway and it was taken for granted they were going to bed African slave women it wasn’t until the prospect of white women, even if Irish, being given to black men entered the equation they bothered to outlaw black:white relations, and of course even then it was only enforced for black men:white women or if a white man was daring enough to want to marry a black woman.)