What's a good test of a pool drain (whether it's working or not)?

My pool is a bit complex, and, I'm trying to troubleshoot a poorly
functioning (supposedly) self-cleaning system, and, one datapoint
would be which of the 5 drains are working at any particular time.

I put my hands next to them, but I can't feel anything flowing.
What's a good test?

Here, for the record, is a picture of the main drain (yellow) on
the deep end and the main safety drain nearby on the wall (red):
http://www.flickr.com/photos/98287134@N02/9330554473/

Here is a picture of a drain on the wall under one of the skimmers:
http://www.flickr.com/photos/98287134@N02/9330560893/

And, here are two drains on the bottom of the spa:
http://www.flickr.com/photos/98287134@N02/9330564727/

Given there are five drains, and three pumps (filter, cleaner, and
spa), it behooves me to figure out which drains are actually working
properly.

Hence the question:
Q: How best can we tell whether a drain is working properly at any
one point in time?