It can be hard. If they prattle on about their "lineage" I tend to shy away. A Tai Chi sifu should be pretty calm and rather retiring. Beware guys with black eyes
Tai Chi is taught in a ton of places. There are a lot of videos on youtube that show very authentic versions of the various forms (24 movement, 32 movement sword, 40 movement, etc). Since Yang style tends to be slow and non-combative at least you're not at risk of getting hurt (unlike some MMA schools taught by bozos). You just need to watch for hyperextension and use proper stretching (which should be part of class).
Tai Chi is also associated with some of the "new age" types, and whether or not they know what they're talking about is up for discussion. Some have studied with a proper sifu/school, others picked it up somewhere and make stuff up.