How long have they been in business? Check their reviews on Google and Yelp.
Ask them to email you a contract so you can review it thoroughly.  Ask about their monthly fee for the service.

What is the cancellation policy on the contract?   

What monies will be refunded?
Ask them to explain their method of negotiation.   Is it with one creditor at a time?  What happens to the other creditors during this time?  Can other creditors report defaults on your credit reports? Can other creditors sue you?
Ask how the settled debt will appear on your credit report and how long it will stay on your credit report.

Ask if there is a ‘stay’ on creditors contacting you, similar to the one in filing for Bankruptcy?
Ask what the advantage of hiring a debt consolidation company is over filing for Bankruptcy.

Ask them about debt forgiveness.

Ask them when your credit score is expected to increase and when.

Consult with a Bankruptcy Attorney and have the pros and cons explained to you.