The only way to really tell is to check it with the Pythagorean Theorem.
The Pythagorean Theorem describes every right triangle, and every triangle that fits the Pythagorean Theorem has a right angle in it. Guaranteed. This is incredibly handy.
If this IS a right triangle, then
(one leg squared) + (the other leg squared) = (the hypotenuse squared) .
Let's check it out:
-- One leg squared = (11 cm)² . That's 121 cm² .
-- The other leg squared = (√99 cm)² . That's 99 cm² .
-- Their sum is (121 + 99) cm² . That's 220 cm² .
-- The hypotenuse squared is (15 cm)² . That's 225 cm² .
Is 220 cm² equal to 225 cm² ?
NO ! They are not equal. They are different !
The sum of the squares of the legs is NOT equal to the square of the hippopotamus.
==> This triangle does NOT fit the Pythagorean Theorem.
==> It is NOT a right triangle.
(The angle in the bottom corner is slightly bigger than 90°. But you can't tell by looking at it. It's so close that you can only tell by checkoing the lengths againt the Pythagorean Theorem.)