It's a roll of the dice. They may or may not detect marijuana, tests of this nature are very imprecise. I'm not kidding, they are. Cannabis use may not be detectable after a week, or two weeks, or a month. There are too many unpredictable factors.
All you can do is go ahead and do the test... and what will be will be. Whether you worry about it or not, that is still what you will have to do.
One thing you could possibly try doing is to get lots of exercise. THC from Marijuana can be stored in your fat cells. If you exercise and burn them off, then there's (theoretically) less of a chance that it will be detected in your system.
There is also the possibility that a small amount of THC in your test results won't bother them, as long as you aren't positive for meth or crack or anything really troubling.