The best way to be sure of the plugs is to check amp draw at each plug. Put an amp-meter inbetween the power lead and the plug and try one at a time.
An ohm-meter can give you false readings, and a voltage reading will not tell you if the plug itself is actually working. Any plug that is making heat has to draw amperage.
As far a cycling goes, all depends on what system you're using, what brand plug, etc. Some alternative plugs draw different amperage and have different heat times. You should always try to start after the first round of whatever (9-14 seconds). If no go, turn the key all the way off, and the back on to recycle (if a civilian system). The main thing is, the engine needs red hot plugs to fire. At first, it takes a plug anywhere from 9 to 15 seconds to turn red hot (depending on brand and spec type). Once it's cycled and been off for a spell, it won't take as long the next time to get as hot. Takes some guesswork unless you've got a good automatic system with temp-draw sensors working. Some off the road equipment does it the easy way. Puts an extra glow-plug on the dashboard so you can actually see how hot it's getting, without counting seconds. Just keep in mind that some el-cheapo brand glow plugs will blow to pieces if heated longer then 10 seconds straight.