Ask the builder about any liability and compensation insurance he may carry to protect you in the event of an accident during the construction of the pool. There is nothing worse than getting into a situation in which things didn't go as planned and not having recourse for receiving compensation because of damages. Every reputable builder should be fully insured. Period.
If you ask and do not receive a copy of worker's compensation and liability insurance, do not use the builder. You may even go as far as to call the agent listed to verify that their insurance is in force. Finally, remember worker's compensation protects people. Liability protects property. You need to have both.
Other things you might consider are the builder's credit rating and you may want to check county records for lawsuits.