First, if you can, inquire with the groom’s family (or whoever is hosting) when you RSVP.  If that isn’t an option, take into account where the rehearsal dinner is held.  If it is at a nice restaurant, opt for a more semi-formal approach: women in dresses or nice slacks and a dressier top (no khakis!), men in slacks and a collared shirt with a sports jacket.   Often a rehearsal dinner is far more casual than the wedding reception, such as a BBQ.  In this case, dress respectably casual.  No holes, wrinkles, sleeveless shirts, or logoed t-shirts (really, leave the Hooters tee for another time).  Most often, people wear whatever they are already wearing to the wedding rehearsal.