Well, I don't know what tests they would use, but I'm assuming they would use the same chemical hCG tests that physicians use to check for pregnancy. The sensitivity of the tests (especially checking the blood) is very good, as it has to be to detect pregnancies early, since the levels of hCG in a woman's blood are fairly low at that time. To the best of my knowledge, the most sensitive tests can detect levels as low as 5 mIU of hCG per mL of blood. That concentration of hCG (equivalent to 5 IU per LITER of blood), is far too low to produce any significant stimulation of the Leydig cells. Just compare, we're talking about 25 IU of hCG in the ENTIRE blood supply, and the typical dose of hCG is what, at LEAST 500 IU a week?
Now given that the half-life of hCG is pretty long, and that there are about 5 L of blood in an adult, and assuming your friend is running about 1000-2000 IU a week, we're talking a sizable amount of concentration in the blood after running it for a few weeks. It's going to take some time to clear that much hCG to get below the 5 mIU/mL threshold.