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#dc, 7

?dvd*-p

Outputs 0 for truthy and an integer > 0 for falsey.

Try it online.

?          # Push input.  Stack: [ n ]
 dv        # duplicate and take integer square root.  Stack: [ ⌊√n⌋, n ]
   d       # duplicate.  Stack: [ ⌊√n⌋, ⌊√n⌋, n ]
    *      # multiply.  Stack: [ ⌊√n⌋², n ]
     -     # take difference. Stack: [ n-⌊√n⌋² ]
      p    # print.