'3.4' isn't a number?
'-3' isn't a number?

IF EMPTY( CHRTRAN( MyTable.MyField, '0123456789.-', '' ) )
  *** This is a number
ENDIF

Of course, this does not take into account the position of the '-' or whether or not there are 2 '.' in the string. But you have received some other advice on how to do this <s>.