Pretty easy to find the express trains; just go to bahn.de and look for trains with 42 minute running times. The others run 44 minutes.

It appears every other train is an "express" on weekends, and waiting for one will delay your arrival time by only 2-5 minutes over the earlier regular train. On weekdays, "express" trains don't start until 9:30 or so and a lower ratio (express to regular). Look for times as low as 38 minutes to find them.

Note that the express trains don't save much time; they can't overlap the regular trains running on the same tracks, so their top speed is slower, they just don't stop. For the tired traveler, I'd definitely recommend the express option, since you won't have to be as watchful at each stop. Safe as RER has been for me, if you were a pickpocket, you'd certainly prefer a train that you can get on and off in the middle of nowhere.

I wasn't aware of the specifics until now. This is really useful information!

For train, RER & metro schedules, bahn.de is your friend.