Here are some ideas to start with:
If the school offering a job has no website, forget it.
If it has a website, try to judge from the site whether the school appears to be genuine. Are any suspicious claims made? Is there an opportunity to contact present/past teachers?
Make sure that they offer you a contract before you think of joining them. Read the contract carefully. If there is anything you are unclear about, write and ask for clarification. How quickly/helpfully do they respond to your questions?
Under no circumstances part with any money for a 'job-finder's fee'.