I have no idea where in the US you are located or what your needs are, so there is very little I can offer you. Personally, I would rather find a place that is local where I can drive it. It sounds like this is your very first time going to an outside lab or you wouldn't be asking. So, start with the basics. How many days do you need the lab. Some may charge by the hour and have a minimum. Are you trying to get something certified, running pre-compliance or maybe just some sort of engineering quick check. I suspect there are many labs that will not be able to certify a product. If you are there to just try some things out, I would bring what every tools you feel you need. Soldering iron, caps, beads, foil. Don't count on the lab to have what you need. Make sure you have a clear plan in place before you go with clear goals. You may find the cost to rent a lab an eye opener. You don't want to be wasting time there. Make sure they can meet what ever standards you require. I suspect many labs may only be able to run low power levels. Their chambers may not go low enough. Keep in mind that even though the labs are required to calibrate their equipment, there is quite a large variance lab to lab. Make sure you account for that. Personally, I like knowing the margins and tend to push my designs to their limits. I have never felt the need to visit a lab before using them but I do spend a fair amount of time upfront making sure we are in agreement as to what our goals are and how we are going to accomplish them. Get your list of questions together and start calling a few of them. They are their to help.
Good luck with your project.