This is a difficult question to answer without knowing how savvy you already are with real estate finance, but there are certainly some basics I can offer up to get you started:
1. How many points? Meaning, how many percentage points of the total amount you're borrowing do you have to pay to the lender as an origination fee? Hard money lenders are notorious for high points, like in the 3-10 range.
2. What are the borrower qualifications? Some hard money lenders are willing to be pretty lenient with respect to previous foreclosures, bankruptcies, poor credit, etc. Ask them for an open, honest answer so you don't waste each others' time if you're a "nontraditional" borrower and fall outside their comfort zone.
3. How long does it take before they can fund your deal? Hard money lenders can usually move much more quickly than a bank, but they all vary.
4. Do they require you to personally guarantee the loan even if you're buying through an LLC or corporation?
5. If you're a new investor, how willing are they to be patient with you? Is there any mentoring aspect to this or are they a silent partner?
Hopefully others will chime in! Best of luck to you.