There are many things that you can do or verify to be extra careful when dealing with a contractor on a large project.
You can ask for many things, including:
1) Business license for the main contractor and all subcontractors.
2) Contractor’s license for the main contractor and all subcontractors.
3) Proof of insurance for the main contractor and all subcontractors.
4) Proof of bond for the main contractor and all subcontractors.
5) Proof of worker’s compensation insurance for the main contractor and all subcontractors.
6) A list of everyone who will potentially have lien rights, including all subcontractors and suppliers.
7) Lien waivers from anyone who would have lien rights on the pool as they are paid.
8) Quality control specifications for all materials and workmanship. For example, what constitutes “level” on a pool (+/- 1mm or +/- 2”)?
9) References for previous work completed.
However, coming at a contractor with all of these requirements can be somewhat off-putting as many customers don’t do this.
There will likely be contractors who would choose to ignore you if you approached with these requirements.
At the end of the day, the contractor’s integrity and reputation will be what determines the outcome of the project.
You can ask for any or all of these things if you feel like they are what you need to feel comfortable about the job, but they don’t guarantee anything, really.
In my opinion, all of these things are reasonable to verify, but many contractors would likely just decide that you’re going to be a difficult customer and not worth dealing with.