Mainly, freezer burn will look like a sort of leathery patch. On beef, it’ll be grey or brown. If it gets to this point, it’s definitely not worth eating in my book. I don’t even bother trying to cut off the damaged parts because I can taste the difference.
Lesser cases of freezer burn can sometimes be identified by ice crystals inside the package, but you have to know how it was packaged originally. If it was packaged wet, then the ice crystals will be there either way. However, if there was no ice to start with (like most commercially packaged frozen items) then the visible ice crystals are your first warning sign that freezer burn is starting. (After all, freezer burn is literally the result of moisture in the food being pulled out.) Items in these kinds of cases are probably still good to eat, but you kind of have to judge them by how long they’ve been in there and how well they were packaged.
Keep in mind that freezer burn doesn’t mean that the food is dangerous in any way. It’s just not going to have the flavor and texture that something more fresh will have.