Each piece would have to be examined carefully. If they were legitimately painted when the item was relatively new and unworn, there should be signs of "compatible" wear. For example, if the knees are faded/damaged then the paint should not be as strong/bright/solid as sections that have significantly less wear (like the backs of sleeves). Likewise, if the uniform is overall significantly faded/used the pain should not appear bright/fresh overall.
It's hard to tell from pictures, but if you look at my jacket's chest pockets you can see that the orange/brown color is not uniform. Edges on the tops of the lower pockets is also a bit worn.
Still, there are reenactors out there who make such things and then wear them a lot. Enough that they could cause the paint to be significantly worn. So my thinking with such items is:
1. If there are few signs of compatible wear, it's probably a fake
2. If there are signs of compatible wear, it might be legit
3. The only way to know for sure is to have solid documentation of the specific piece in question
#3, unfortunately, is a tall order so #2 is probably the best we can do.