NOTE: There have been blood tests since 2004 for HGH, disregard the above.
There are already urine tests to detect HGH. The NBA doesn't use them though.
How reliable is it, though? That's the reason that the various sports unions (MLB, NFL) have refused to agree to a test for HGH. Thus far, the owners aren't forcing the issue, and seem to agree.
The urine tests aren't very reliable yet, although I think they'll be very soon.
However, the blood tests are extremely reliable - WADA introduced them in the 2004 Olympics Games, if I'm not mistaken. Why don't the unions agree with blood tests? Nowadays, it's impossible to catch cheaters without blood tests.