Even lower tech: scotchtape a small piece of toilet paper to the front of the speaker so it flaps. You think I'm kidding? I did that once while screwing around with modes...my backs are mounted in the "Dolby locations" which is somewhat over my head, so unless I turn off the surrounds I can't always tell if something is coming out the backs unless it's loud.
It's handy to see your backs are working (without having to get up) until you're confident that your decoder etc. is doing what you think it's doing. There is usually not a lot of sound in the backs in a proper EX encoding, so you could end up standing by them a lot until you find some action there without a "visual aid". (dts ES discrete soundtracks are often a different matter, some have quite a lot going on in the backs.) I suggest removing the toilet paper EX flags before guests come over, but it's your call...

The THX trailer is properly flagged, I don't think I've ever encountered one on an EX disc that isn't (some you can check with are Star Wars, Fight Club, Titanic, Evil Dead, Terminator 2, Incredibles, Monsters Inc., Finding Nemo, etc.).
If you force the EX mode in an EX-capable decoder, it will be decoding it "properly", or at least as EX requires (the back sound comes out of the surround channel info). The flag does nothing more than allow your decoder to auto-detect, it does not in itself change the sound, just indicates to your receiver what is there (I like to be the boss of my sound system, and *I* choose the sound mode, that way me and the equipment see eye-to-eye).
Whether the sound is matrixed/EX-encoded or not, forcing EX-decoding for the backs will get *something* to them. If the sound wasn't matrixed for EX, you may or may not like what you get. In fact, it is possible to get sounds that will almost fully negate the sound from your surrounds, or at least completely screw up the surround sound field in the seating area of your room.