To find out if your prospective employer is legitimate, you can check and see whether they are on the SAFEA register. To do this, download the PDF, then search for the Chinese name of your employer in it (the schools are organized by province): http://www.safea.gov.cn/content.shtml?id=12746539
If you have trouble searching the document, simply ask your prospective employer which page their company name is on, and check the name matches the one on their website. If they are not on the list or the names don’t exactly match, they are not authorized to sponsor you for a work visa.
One of the most common questions is whether it’s legal to start working on an L-visa or F-visa. Many schools, centers, and especially agencies encourage this behavior and tell people that it’s legal. Let me tell you, it’s illegal to work and receive a salary in China on anything but a work visa. People who tell you otherwise are either misinformed or lying. Many employers try to encourage teachers to start working on a tourist visa (L-visa) so they can see if that teacher is up to standard before going ahead with full sponsorship, or because they are not authorized to sponsor foreigners for a work permit. This situation is illegal and can be disastrous for the teacher. The police do random inspections and catch teachers on the wrong visa. Even if you do manage to work for a while on a business visa, your employer might have trouble getting you a full work permit. In summary, the only places which are truly legitimate are those who offer to sponsor you for a work permit (z-visa) upfront.
The other big myth is that F-visas and L-visas can be “converted” into a work visa (z-visa). A few years ago, this was sometimes possible. Now, it is forbidden by government policy in all but exceptional circumstances. If you’re in China without a work visa, to get a work visa you need to apply for it from scratch, which typically means returning to your home country to apply even if you’re already in China. For those who are already in China and don’t want to apply from their home country, authorization is occasionally given for an applicant to apply from Hong Kong.
The final thing to be aware of are many scammers posing as agents or schools, who will ask you for scans of your passport and qualifications. After you send them the information, you won’t hear back from them. These organizations may send all your documents and information to anybody who’s willing to pay – usually perpetrators of fraud and identity theft. If you don’t want to get cheated, it’s essential to check on the SAFEA register to be sure that you’re dealing with a reputable company.