Load the whole music\artists\artistA folder (or maybe even music\artists) into Mp3tag.
First sort by Genre, then Artist. Have the Artist and Genre columns next to each other and you should be able to scroll down through it, look at all the tracks for one Artist and see if the Genre changes.
btw: what a poor artist that serves only one genre.
On the more serious side:
It is not possible to compare any two tracks with MP3tag.
So there is no indicator that any named field, when compared, has different entries. I would appreciate such a function to find duplicates (so to say the other way round). But is not possible.
You have to find which entries you want and then e.g. build a filter that shows all those not matching your criteria
e.g.: %artist% IS artistA AND %genre% NOT IS Folk.