There are a couple of general rules to follow with procs. Auto powers are the best placement for certain procs like Panacea or Numina. Some procs are global, and will activate when their conditions are met, think Spirit Ward slots Chance for +Absorb - it's always active, but certain conditions need mer for the effect itself to activate. Then there are damage procs and chance for effects like -res and -def, etc. These have certain proc per minute (PPM) numbers between 1 and 4.5. The higher the PPM, the higher the chance for the proc to go off in certain powers, which follow a formula based on Auto/Click/Toggle and single target, AoE and Cone targeting.
When in doubt though, you can use one of the many handy PPM calculators that members of the community have made. I'm sure there is a link someone can share, but I use one that's in a spreadsheet. Let me see if I can find the link...