You taste it. Most nicer tea shops will make you a free cup of several types of tea, go with the one you like most. There's all kinds of fine teas in China and you'll just have to go with your preference

Or you could ask them and I'm sure they'll tell you what's fashionable for rich people, but remember that will be super expensive and the tea equivalent of an iPhone or a Justin Beaver haircut.

Checking to see if there's mold in it is always a good first step.

To be honest, I cannot tell the difference between tea qualities. To my unsophisticated palate, they all taste the same so I trust my girlfriend to steer me true.

It's like wine, you can get an idea base off of smell, appearance, and where it comes from.  But you'll never really know until you taste it.