It's not necessarily true that one should avoid any school that withholds a certain amount of your salary during a probationary period.It is a common practice at some universities, and rightfully so considering that many teachers are not suitable for long term positions and it is difficult for them to make that determination wihtout witnessing the abilities of said teachers. However, it is true that one should be suspicious of private schools that attempt to do so - for any reason. In fact, private schools should generally be avoided. There are a few exceptions of course.
If you're thinking of working for a private school, it would be wise to avoid those that do not have Western managers or owners; however, you should be initially suspicious of any private schools, and of course do your research.
Good luck!