If you are looking at a kendo dojo, they should be affiliated with the International Kendo Federation through the local federation. If you are looking at a iaido dojo, then a similar association through the I.K.F. would be one sign of legitimacy, or else a link to the Zen Nippon Iaido Renmei. Not all iaido dojos belong to one of the two federations, though. But in some isolated areas that is not possible and you may find a junior person doing the best they can. If you have your heart set on kenjutsu or iaijutsu you are probably out of luck.
Instructors are few and far between.
If you find a school, be cautious - there are frauds about. Be especially wary if a lot of money is being charged. Kendo and iaido instructors are always volunteer and most legitimate kenjutsu instructors work the same way. Similar to unaffiliated iaido dojos, find out what the ryu is, what the instructor's qualifications are and who his teacher is. If you get unsatisfactory answers or the questions are being dodged, don't join.