Here in the UK, they tend to call themselves Consultants.
I would be a little wary though as anyone can claim they are an SEO consultant and its very hard to check their skill level. They never offer guarantees either and if they do I would run a mile.
If possible I check out some references and preferably not friendly clients of theirs that they would like you to contact.
I would also start them off on something simple and make sure you have a get-out clause written into any contract if they don't deliver as promised.