I measure across the diagonals, if they are the same then the quilt is square. If not I keep pulling it gently along the sides a little at a time until they are and (if necessary I pin it in place) then leave it to dry with fans on for a day with wool and polyester battings, two-three days for cotton depending on how humid it is.
If it isn't a show quilt, but it looks square anyway, then I don't bother.
HTH