Have the dealer clearly explain what is covered and what isn't. A reputable manufacturer will cover not only the spa shell, but the plumbing, jets, and all major components for up to five years. A good rule of thumb is to buy a spa that has been made by a manufacturer who has been in business longer than the warranty they offer. Perhaps the most important feature is how we feel sitting in your spa. It's also important to get a feel for how a particular model feels wet, and to do that you have to climb in when there's water in it.
Spas feel very different wet than they do dry, as many spas are displayed.
You wouldn't consider buying a car without a test drive, and you shouldn't consider a brand of spa without a test soak. In most cases, simplicity is the rule. Make sure you understand how to operate the spa, and strive to eliminate unnecessary features that are there just for show. You want a spa that is ready to use, and that works every time you want to get in it. No troubles, no hassles.