If I am paying upfront, is it legal?
In certain states (California and Illinois, for example) it is illegal for a company to charge upfront fees to negotiate a loan modification for you. In California it is illegal for an attorney to charge upfront fees to negotiate a loan modification for you - even if they are giving legal counsel. So is your negotiator charging you up front? If so, is it for the analysis of your situation, packaging your proposal to the lender, reviewing lender docs - what specifically is the fee for.
What am I paying for?
You may be paying for legal services or for an analysis of your loan documents (many loans written in the mid-2000's were pushed through quickly and incorrectly), advice on how what to expect from a modification. You may be paying for the expertise of knowing what lenders want to see in the documentation (how do you balance letting the lender know you haven't been able to pay with your ability to pay going forward?). Read through your retainer agreement - No retainer agreement? Then what ARE you paying for.
How do I know I dealing with someone legitimate?
Places you can check - the Attorney Registration website for your state, the Better Business Bureau, the Attorney General's office for your state.