How can you tell if wxy is a right triangle?

Answer 1

If the triangle contains an angle whose measure is equal to 90 degrees or if the sum of the squares of two of the sides equals the square of the third.

Essentially, if it has a right angle or if a2 + b2 = c2 where c is the longest side of the triangle and a and b are the other two sides, then the triangle must be a right triangle (in Euclidean geometry, that is)