I happen to do this for a living(utility locator). You should have a One-Call system or similar agnecy that will take a toll free call from you and alert all the utility companies and they in turn come within a predertimined time period(usually 3 days) and mark all of your underground utilities.
I would not sign any co-liability agreement with anyone. Although you may know where YOUR lines are you don't know what else is there like your neighbors utilities, the buildings or houses behind you, mains, primary lines, transmission lines etc. However it is unlikely but you would be surprised how many residential neighborhoods have gas transmission lines, electric trans lines, fiber optic trunk etc running right through back yards! As a matter of fact we have tons of Texas Eastern gas lines in my district.
Be careful at what you sign. Repairing the line is usually not the total damge cost. In most instances there is down time for the contractor, sometimes cost of gas volume lost or cost of electric service outage, possibly repairs to taps and mains if they were pulled apart or seperated, there's lots that can happen. What if the guy with the auger get electrocuted and is burned real bad or even dies? You wanna pay have of the settlement? Believe me our company had to pay a McDonald's 8k for one simple service hit after all was said and done and the repair to the line was only $500! Today we had a primary 34kv line hit with an air spade and the operator is lucky to still be here. Sounds like this outfit is reckless and has had to pay before. It is not your responsibility if you are contracting him to dig. Get in touch with your local utility providers and ask if they have a One-Call or simailar system. If not hire a professional utility locaor to mark out your lines and don't sign anything because Murphy is alive and well!
Edited to add the Texas One Call #'s 1-800-344-8377 or 1-800-245-4545. They will have all the answers you need but make sure the CONTRACTOR calls this in! Ask him for his ticket and do not let him dig before it is valid. Again don't sign anything.