Lorna Mitchell has posted a quick checklist of things you can ask about your API to see if it's RESTful or not (five of them):
I get a lot of emails asking me to get involved with API projects, and that means I see a lot of both implemented and planned "RESTful" APIs. [...] A service of some other description may work better for other scenarios or skill sets, and non-RESTful services can be very, very useful. If you tell me that your service is RESTful, then I expect it to be. If you're not sure, look out for these clues:
- It has a single endpoint
- All requests are POSTs
- Response metadata is in the body, not header
- There are verbs in the URL
- The URL includes method names
She suggests, though, that "being RESTful" isn't a requirement for "being useful" when it comes to APIs.