Southwest used to be pretty accommodating...but since the introduction of the A, A+ Standby policy they have been pretty strict on not allowing standby. I have seen first hand quite a few people be denied standby even with plenty of seats open on an earlier flight. "Pay the difference in fare" is the new line.

So unless you are A List, A+ or there is a delay in the system that will affect your flights your chances of just "hopping on" another flight are pretty slim, but it never hurts to ask.