I'm assuming that you're referring to the Japan Karate Shoto Renmei?
If it's a group that traces its lineage back to Asai Sensei, then they're the real deal.
Just as a bit of background (any number of online bio's will also state this), Asai Sensei was one of the higher-ups with the Japan Karate Association, and also one of its technical directors, so he had an excellent eye for technique, fundamentals, physical training, flexibility, adaptability, etc.
If the group you mentioned adhered to what Asai Sensei's methods, then they should be a technically sound one.
I had the pleasure of training with one of the people who was under Asai Sensei, Kos Yokota, and came out of that seminar with quite an appreciation for their teachings. He really put a focus on how you can optimize bodily mechanics.
For that matter, even though Yokota Sensei is in his 60's, I would swear that he's in amazing physical condition, and has the physical abilities of a man half his age...
I would second what Grenadier said. JKS is a very reputable shotokan organization, with famous instructors like Kagawa Sensei and Yamaguchi Sensei. The late Asai Sensei was the senior JKA instructor when Nakayama passed away, and was probably next in line to be come director, but he had a lifestyle away from the dojo that did not agree with some of the more conservative members of the organization. When he was not appointed director, he left and eventually formed the JKS. There was never a question about his karate, though. He is prominent in Nakayama's Best Karate series. So if your prospective tenants are members of the JKS, then at least their organization is real, mostly present in Japan and Europe I think.
Thank you gentlemen. I spoke to the instructor last Friday. He pays regular visits to the Hombu in Japan, attends courses there and is even ranked in Kendo. Seems fine to me.