Hi , How do I find out if its an right triangle?

Does it have a right angle (an anlge of 90°), then it's a right triangle.

If you know the sides, use the Pythagorean theorem:    a² + b²  =  c²

Choose one of the two smaller sides, put that number in for  a;

take the other smaller side and put that number in for  b;

put the largest number in for  c.

Calculate the left side (a² + b²) and calculate the right side (c²); if they're equal it's a right triangle; if they're not, it's not.

When one angle in the triangle meet is a ninety degree angle, like the corner of a square