Check out any reviews online. If you see them all over the internet you can bet your hard earned tattoo money they are probably very popular. But this makes your chances of getting a booking in the next 2 years all the slimmer!
Only listen to advice of people who have actually gotten work by the tattoo artist in question. There is nothing like first-hand experience. This is not to say that you will get off on the artist the same way your friend did, particularly if she is female and the artist was a dude!
Seek out a tattoo artist who specializes in the style of tattoo you are about to have done. If you are looking for some Polynesian style black work and know a particular artist that specializes in this style you should go to them.
There are many more considerations to keep in mind not LEAST the standard of professionalism and cleanliness they portray in their studio. All tattoo artists have a legal obligation to maintain a high standard of practice by using sterile autoclaves and needles. Always check their credentials and if they are a member of any professional associations. If you are in any doubt at all DO NOT get tattooed there. Not only will you risk the transmission of blood borne pathogens from amateurs also known asscratchersbut ultimately you WILL end up with a really bad tattoo.