Without experience that would help you recognize it right away, I think you would just need to try numbers until you find it.
Pick a number, multiply it by itself with pencil and paper, and see if it works.
(2)² = 4 too small (3)² = 9 too small, and not growing fast enough; this could take all day (10)² = 100 still too small (15)² = 225 oops, too big; go back down (12)² = 144 too small (14)² = 196 too big (13)² = 169 just right; hope the bears don't come back too soon
