We don't know your son and his partner so we can't tell you whether he's using, but the fact that he's gay definitely doesn't correlate with a likelihood that he is using crystal meth (or any other drug). Believe me, the vast majority of us don't. Just like the vast majority of straight people don't. Yes, there are statistics to show that gay people are more at risk to abuse drugs for the reasons spelled out by the above posters, but you must remember that those using drugs are still most certainly a small minority of the gay community.
Now, to answer your question, warning signs of addiction to meth would be extreme thinness, bad teeth, way too much energy, not sleeping... do a Google search for meth addiction and you'll find out more. Meth is a terrible scourge; it ruins lives and peoples' future.
Once again though, I want to emphasize that suspecting your son and his partner are using meth simply because they are gay men living in San Francisco is an unfair correlation. Unless your son has abused drugs in the past or there are other facts you haven't told us I'd suggest you forget all about this and have a great time in SFO with the two of them!!