Well I have to say first and foremost that this is a rather disgusting thread about the subject of dirty nappies. I have very limited knowledge of this area and I suppose it is similar to alot of people who may have seen someone who is a parent basically having to sniff around the baby and around its bottom to see if it has a dirty nappy. And is there any way of telling if it has a dirty nappy without doing something like this alot? Also if there is a factual answer to other possible ways of telling this?
Sometimes you can catch the baby in the act. Apart from that, there’s sniff test, the squish test, and the sample test. And the dreaded blowout, but that’s a giveaway, not a test.
This article suggests several other tests:
- The squeeze test
- The peek test
- The finger test (which I’ve also heard called “the dipstick test” :eek: )
Modern disposable diapers have a line on them that changes color when they get wet.
I can’t think of an alternative better than the sniff test.
When we suspected a present awaited us, we tapped the bottom of the diaper. The extra weight would be quite noticeable. (Esp. after gaining the experience of changing a few hundred diapers.)
When in doubt, it was the peek test.
(Note: we used cloth diapers almost all the time. YMMV with one-use ones.)
Given the OP’s references to “dirty” nappies, and having to sniff around the baby’s bottom, it seems clear they are inquiring about the presence of feces rather than urine. I suppose if the baby has severe diarrhea then there might be enough moisture to show up on the wetness indicator, but this seems like an exceptional case.
The lil’wrekker told me. She was always the ‘princess and the pea’ type baby. Things had to feel right or she protested.
Before she could talk, how would you tell if she needed a diaper change versus just normal crying?
~Max
A parent knows, Max, just because an infant doesn’t use words, that doesn’t mean that they don’t vocalize with different cries for different things.
Dad of 4 and very experienced changer of diapers and feeder at midnight and 3 am
As Dork says. Mom, Dad or daily caregiver knows the diaper cry, cranky cry or hunger cry.
Yep. The “where is everyone” cry. There are quite a few cries.
The “who are you and why are you holding me” which leads to “i want my parent” which can very quickly morph into “I want THAT parent”
Babies have a pretty good vocabulary, just not words.
Funny story; when Mid-daughters first born was 8mos. old he caught an upper respiratory virus and was put on prescription meds. She had a little spoon thingy to give him the liquid meds with. One morning he was in his little chair and she was pouring his meds up. He really hated the meds. She swears his first words were “no medicine Momma”. I think maybe sleep loss and the 1st baby syndrome was in play. She’s convinced he did it.
The lil’wrekkers first communications were alot of cooing, goo-goo,gah-gah and pointing and hand signals. Almost like signing. Her first word was fire-truck only she said ‘fire-fuck’ 
The baby knows.
And believe it or not, many of them do not like sitting in a bag of shit until someone else notices. They tend to tell you in their pre-lingual ways.
Sometimes you can tell by the expression on their face. It’s a sort of inward focus. But as mentioned, they will let you know.
Regards,
Shodan
My nephew did that. It was very quickly dubbed “poopy face” by his mother. 
Well, Pampers is now offering a “smart diaper” with a built-in sensor that sends an alert to your phone when your baby has soiled it. No, I’m seriously not making that up: Did Your Kid Pee? This Pampers Smart Diaper Will Let You Know | PCMag
I figure stretching the waist out a bit, looking down there, and voicing an evaluation is the gold standard. That’s what strangers near me in restaurants are always doing.
The diapers are sensitive enough to pick up moisture from regular poop too. It doesn’t require diarrhea of any kind.