Build yourself a variable, something like the following should do,
=Pos(Trim([First Names]); " ")
It Trim(s) any leading and trailing spaces from your First Names column and then returns the Pos(ition) of the first space it finds in the column. So any result other than 0 indicates two or more names in the column. Once you can identify the rows you're after there are a number of options (which I can't currently remember the details of) open to you.