How can you know if the photographer does approved passport photos?

When it comes to US requirements, normally most passport cameras will do the trick.  The US is not as difficult to deal with as other countries.  I would not recommend anyone such as a drugstore or big box store to do foreign passports or IDs.  The sizes, backgrounds are all different.  You need to convert to metric in most cases.  I no longer use my passport camera because it was so much guess work.  I use the same studio camera and Mac system that I use for professional portraits.  The other countries can also be really picky and will turn it down for a slight shadow, reflection, size off  a few mms and so forth.