The converse of the Pythagorean Theorem:
If c^2 = a^2 + b^2 then the triangle is a right triangle
c = longest side, a and b are the other two sides
c = 65, a = 16 and b = 63
So
c^2 = 65^2 = 4225
a^2 + b^2 = 16^2 + 63^2 = 256 + 3969 = 4225
In this case
c^2 = a^2 + b^2
4225 = 4225
Triangle ABC is a right triangle