Caudopunctatus are easy (look at caudata culture). Males have spots on tail. Tail looks like a pachytriton's. They are usually much lighter colours with a greenish back.
Chinensis - two forms, one is a putty coloured/oliveish colour, the other is black. They often have yellow bellies and yellow spots at the point where limbs meet body, sometimes on dorsum as well.
Fuzhongensis - has very warty skin with 'pointy' worts as opposes to the more rounded glands of chinensis. They have a fleshy overhanging upper lip below the eyes and the tail goes mottled with brown/black at the end.
Hongkongensis - smooth skin, distinct cranial ridges. Males often have smaller spots with greenish/black rings around the spots. Sometimes have red irises.
That's basically the species you are likely to find. Guanxiensis are almost identical to fuzhongensis, and the only real ways of being sure are proportional skull measurements, DNA and disecting various bits of the body.
In my opinion/experience, hongkongensis are much less hardy than other paramesotriton.
Caudopunctatus tame more quickly and will beg for food more than other paramesotriton.
Look at
www.caudata.org/cc at the pics of paramesotritons, there are lots of new pics whih are much better than before.
The one thing with paramesotiton is YOU CAN NEVER BE ABSOLUTELY SURE!!!! There are possibly separate species/subspecies with caudopunctatus and there is a large possiblility of unidentified species, as with pachytriton, coming into the pet trade. Buying form the same shipment isn'r foolproof as the animals are often mixed up with other localities at wholesalers.
Look for sores on the mouths and backs, and any other abnormalities. A frantically active newt is probably not healthy, as is a subdued one.
Also be aware that males are often very aggressive.
Females tend to be plumper with long 'low' tails, whereas males are smaller, skinnier and have proportionally shorter tails with higher 'fins.' In breeding condition males of most species (except caudos) have a white/blue stripe on the tail. All reproductively active males have a swollen cloaca.
Where is this petstore? Are the animals there or do you have to order them in?
good luck!
Chris