i have seen that girl before
it is the correct way to speak because u are talking about the girl to whom the listener can see. and if u say
I SAW THAT GIRL BEFORE
its absolutely wrong because this sentence talks about that girl is still not available and listener doesn't know her coz cant see her at the time of speaking coz PAST talks about action made and ended in past BUT Present Perfect talks about the action made in the past and just stopped..
meant that girl is just gone from where u were so listener has seen her ergo u said THAT GIRL means listener has come to know WHO you are talking about.