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