How can I tell whether an SMT/SMD Capacitor is oriented with plates horizontal or vertical?
You can (or anyway, historically could) buy specially marked ones from ATC (American Technical Ceramics, and now owned by AVX, which is in turn owned by Kyocera) if you want to do this. Or simply choose capacitors that are not square in cross-section, so the orientation can be distinguished even without markings.
Practically, most MLCC's are not marked for this, and are probably loaded in their tapes the wrong way, and we just accept the reduced performance in return for lower cost (because the parts can go into the tape in the same orientation they are diced out from the greenboard they are manufactured on).
If you want to do this with unmarked parts you will probably have to do hand assembly and just be careful to turn them 90 degrees after taking them out of the tape (and tossing any that you lose track of the orientation of).