Let me say first off that I'm not an expert in regards to purchasing a hot tub. I am looking right now. Here's a list of things that I am following myself.
!. Research the dealer and manufacturer, how long have they been in business. Search the internet to see if you can find reviews about them good and bad.
2. Check the Better Business Bureau BBB.org website to see if the company is listed if so check for any complaints to see how they have handled these. I would check both the dealer and the manufacturer of the hot tub.
3. Look at the dealers website with a fine tooth comb. If you look carefully you will find subtle clues. For example, I have a dealership close to me the sell spas and billiard tables. The website is big on the billiards. Their hot tubs are no where to be found on their website. I went into the store and spoke to the sales rep there and asked a lot of questions. They do not staff a team to do repairs on their hot tubs. They consult with a separate company for this.
4. Ask about the warranty at the dealership, after you have looked it up yourself online with the manufacturer. Notice the reaction you get when you ask this question. Do you get a straight answer right off the bat without the salesperson having to look it up. I would be cautious of any warranties under 5 years, that tells me that they don't believe in their products.
5. Take your time look around and ask a lot of questions. I recently went to a dealership and the sales rep knew less than I did about the product without consulting with another person and looking up information in the company manual.
6. After you are definite in your selection of hot tub. Get to know very well, every aspect of it. Your investment in time spent doing your research will pay off in the long run.