That may be so, but they are not going to put it back together, you have legal rights, but is it worth the time and money?
Yes you can remove the radiator and have it pressure tested, but dont waste your time or money.
If you had water in the exhaust and were looseing coolant causing the trk to overheat and no leaks on the ground you had a blown head gasket. They were on the right track but they didnt get your written permission to do the work. Your problem now is finding a shop to put it back together without having the experiance of taking it apart. If you have it towed somewhere else you run the risk of parts getting lost and the new shop having to "fix" what the other shop "screwed up", in otherwords...it will cost you more to have the job finished elsewhere.
At this point all you can do is talk to the shop manager/owner and tell him how disappointed you are and see what he is willing to do.If he wont do anything then play nice, have the repairs done, report them to the Better Buisness Bureau and maybe talk to an attorney.
If you dont play nice you run the risk of them screwing up your trk and them not honoring the warrenty on their work.
Most states have a minimum dollar amount of work that is allowed to be performed without written permission, it varies from state to state.
Sorry, but you are hosed.
If you cannot afford the repairs then have them finish the work and then take them to small claims court. Just make sure that you did/are not signing a estiment or work order but only a completed reciept.