Hello, limubai2000!
I'm not trying to be flip, but in my opinion, it's hard to say what should be in an ad. Hopefully other saloonites will offer something substantial to you. I think it's more useful to talk about "red flags" in ads, such as when they say things like "students age range from 5 to adults" -- BAD sign -- or "must be flexible" which indicates the place is wholly disorganized and unprofessionally managed (which a great majority of schools -- of all kinds in China -- usually are).
You DO want to read that THEY will arrange and pay for ALL medical exams, fees etc., related to your working Z visa.
NEVER allow let them to convince you that an L visa is OK upon arrival and will converted later -- it's guaranteed trouble. Don't accept shared housing. Don't accept "probation" period salary (which can be as much as 2000RMB a month less than the actuall month salary).
Many job ads are posted by agents/recruiters. If they seem genuinely interested, make them put you in touch with the actual employer very quickly. If they refuse, that's another bad sign because the recruiter will often tell blantant lies in order to place you in the position and collect their "finder's fee" -- after that, you are completely forgotten and the agent moves on to the next sucker.
If you get generic and vague replies to probing, but reasonable questions like "Oh, you needn't worry about that," or "No problem" (but no further clarification), avoid the position. This is telling you that truth is not forthcoming, but problems are!
Always request photos of living accomodations and contacts of foreign teachers previously or presently working there. Everything can pose a potential problem. Sometimes, when I have talked by phone with a foreign teacher, the phone conversation was held in the office, with the school's "leader/administrator" sitting next to the teacher.
All sorts of tricks and traps lie in wait... Remain alert and skeptical, and keep asking questions until you are satisfied.
I've made LOTS of mistakes in the past by being too trusting.
If you find a job that you are interested in pursuing more but you are not getting enough information to make you feel comfortable, post your situation here in Saloon.
Raoul and so many other folks here are smart, caring, and generous (and quite witty)!
Also, read lots of the threads here, such as "Things I Just Wont Do for Work" found in the "Teacher's Tip" section of the Saloon. LOTS of useful information posted here now.
Good luck to you!