Let's assume that you have a triangle of sides A, B and C. We know that B and C are less than A, we can replace the values in the pythagorean theorem so that after calculating the sum of B^2 and C^2 it should not be equal to A^2.
Example of a right triangle
Let's assume that B is equal to 3 , C is equal to 4 and A is equal to 5. After Adding 3^2 to 4^2 we see that the result is 25, which is equal to A^2. Then, this triangle is a right triangle.
Example of a non-right triangle
Let's assume that B is equal to 2 , C is equal to 3 and A is equal to 4. Adding 2^2 to 2^2 we see that the result is 13, which is different from A^2 (4^2=16). Then, this triangle is a non-right triangle.