I agree with what ^ he said. Proper movement over speed is paramount.
1. I would ask very specific questions about programming. Since you are an SS guys, for example, I would ask how often they backsquat and how important they consider backsquatting to their overall programming. I say that because some Xfit boxes do not program backsquats. Avoid these boxes. Flip through their programming history.
2. One variable of programming for Xfit boxes is the clientele. If you join a box in the high end suburbs that caters largely to soccer moms, you'll often find the programming to lend itself more to "boot camp" type work outs rather than focus on strength training, power movements etc.
Visit some different boxes and look for some individuals that you identify with. If you find that you are an outdoorsman among yuppies, its probably not a good fit for you. Even if you don't line in a very outdoorsy place, find a box where there are some serious athletes training. Being surrounded by people who pursue their own athletic endeavors exterior to Crossfit will likely fit you much better. Some boxes are only focused on the Xfit games and preparing their members to compete in the games despite none of them having a chance in Hell at even going to a regional event.
3. I would look at their scheduling. IMO, a box should offer the opportunity to work out on any day of the week. Some boxes close on a weekday to have a mandatory day off. I would also look and see how much open gym time they offer. This can vary from 1 hour a week to daily open gyms. Personally, I find this important since I like to use the facility to do my own specific workouts at different times of the year. I'll also frequently work out on my own during scheduled classes, but I've been a member at a place for sometime and nobody can really tell me what to do.
4. Look at the coaches and their credentials. Anybody with a free weekend and $1,000 to spare can become a Crossfit coach. Are they full time coaches or are they coaching classes in exchange for a free membership? Do they hold certifications in addition to Xfit? Oly Lifting, SS, Wilderness Athlete etc would be examples of additional certifications.
5. Also, since you are an SS guy, you'll likely run into some resistance from some of the coaches when it comes to low bar backsquat. This won't necessarily be the case as some Xfit boxes are SS certified and even teach the low bar backsquat, but your average Xfit coach may have never even heard of a LBBS and will try to correct you to do high bar because that is all they are familiar with. I use a LBBS and have run into problems when dropping into other boxes. Sometimes you just have to explain what you are doing and why, other times you just let it go in one ear and out the other, sometimes you just have to tell them to **** off. Some of these coaches are 22 years old with literally 2-3 years of training history and need to be put in their place by people with more knowledge and experience.
No reason to go in with a chip on your shoulder, but that may be an issue you will encounter.