Can I just assume if I need to do a HTTP POST to a url to get back a string of data (JSON or otherwise) that it is a RESTful API? Or is there another term for a general api like this?
I understand that there is probably more behind the scenes on the server that could determine if it is a true RESTful API, but as far as the client can tell, are there any other characteristics?
It might be part of a RESTful API; it certainly doesn’t sound incompatible with one. If it was RESTful, you’d also have extensive use of HTTP GET for doing reading of things (with content negotiation to determine the format — the representation — to deliver the values as), proper use of HTTP verbs for the true meaning (not everything is a POST!) and HTTP error codes for their meanings (not everything is a 200!) and there would be hypertext linking to allow everything to be located.
REST is just a way of writing a rather dynamic website. That implements a service.