What line on the dependent's 1040 is used to determined what the dependent's income is for determining whether it is over the $4050 limit?

On a full 1040 it is line 22. On a 1040EZ it is line 4. On a 1040A, line 15.

what about on 1040 A, is it line 21?

As I said in my answer above, total income is on line 15 on 1040A. Line 21 is adjusted gross income.

Thank you.  I was hoping to use adjusted income for determining dependent's

Unfortunately it's gross income without adjustments.