Best way to check is to look at the trim inside and see if it has the same reveal, or gape, all the way around. If it is even then the slide is square all around. And, if you are talking about the bedroom slide...then it will appear to be angled out from top to bottom, at least on our model year.
The next step would be to check all the seals around the slide for rips, tears, folded or just plain missing. Good luck.
2011 Georgetown 378TS in FireMist
2013 Jeep Wrangler Toad
Kirby the Old Dog, passed but still in our hearts
Max E. Dog, the new pup
Eureka the Old Cat, still hangin' on