Have you been able to prove that the coins are "even"?

I don't think so... You have received several numbers of "Heads" around 50% of the total number of tosses, but as many as you have tossed, it is unlikely that you will get exactly 50% of the coins landing on "Heads".

You will never be able to prove this exactly ; however, the more coins you toss, the closer you will come to a value of 50% of the coins landing on "Heads".

The precision improves like the square root* of the number of measurements. In other words, if you toss the coins a factor of 100 more times, you will improve the precision by a factor of 10 (10 is the square root of 100).

Reminder: the square root of a number N (denoted ) is another number S, that if you multiply it by itself - SxS ("S square" denoted as S²) , you obtain N.
Examples: Square Root of 4 is 2 (2x2 = 2² = 4). Square Root of 100 is 10 (10x10 = 10² = 100).