On my Berhinger pmp 5000 powered mixer the mic trim pots seem very touchy.There is almost nothing on them until they are almost at maximum at which point you get feedback issues.How can I check to see if the pots are operating properly.What should the resistance be?

When using the trim with mics such as SM58's, etc. the trims often wind up at about 3 o'clock in normal use. You set the trims by pressing the PFL buttons down and setting the level using the led display histogram so that at maximum level of talking/singing the display never goes clear up to clipping.
IF you have feedback, then you have issues of mic location and speaker location. The critical point of feedback is far too critical to set with the trim controls.
Your main speakers should be toward the audience from the mics, not so the mics can "hear" the main speakers.
Monitor speakers are ALWAYS a problem with feedback... performers want more monitor, but depending on the acoustics of a stage, reflection from the back can get into the mics or the mics may not be directional enough... SIngers that choose to hold the mic and wave it around are a soundman's nightmare! They let the mic point toward a monitor and away goes the feedback.
The PMP5000 has a lot of power.
While your at it, PLEASE read my tip on FIXYA about PMP mixers regarding the hazard of using Speakon to 1/4 inch speaker cables... ESSENTIALLY never allow the barrel of channel "A" output 1/4 inch to be grounded as it, instead of the tip of the jack is the HOT lead !!!! This isn't in the user manual... I fix these things and discovered this hazard from the schematics.