Not all 757s have DC power. They have been reconfiguring the 75s just like they are doing to the old 73s.

The easiest way to tell is to look at the first class seat map for the 75s. If it is uneven, you have DC which means you don't have power. If the seat map shows an even number of rows on each side then you have AC power and it will work for you.

Similarly, look at the 738 seat map and if the last row in coach is 28 then you have DC power. If the last row is 30 then you have AC power.

Hope this helps!