Are you meant to / allowed to?

[acura,]

Are you allowed to ask a general helper if a trade is fair? Like PM them about it?

[Midde]

I don't see why not, so long you're not asking constantly or asking about the same trade over and over.
If your unsure about a trade you may also want to check out Is this a fair trade thread found under CS Discussion forums.

[Shian]

It’s better to ask on the thread. Everyone is different on cs and the one person you are asking might not have any more knowledge than you do whether or not a trade is fair.

[Burrito Bunny]

As said before by others, I would highly suggest sticking to the fair trade thread. That's what it is there for. We all have to wait a while for trade advice sometimes. c:

If you feel you absolutely must, I would suggest sending in a help ticket instead of randomly pming someone. http://www.chickensmoothie.com/help/gethelp
Although everyone has at least a basic understanding of rarities, some GHs may not feel comfortable answering your trade questions, and if that is the case, you will likely be forwarded into the help system and still have to wait for a long time until the ticket can reach someone who likes to give trade advice.

Basically, you probably won't actually get an answer faster by pming us. You'd be better off sticking to the fair trade thread. ^^