I look for all the classic things bold outlines in Black, smooth shading and blends, make sure lettering in straight and spaced evenly. It is my impression that usually what is in a tattooers portfolio is their best work and or what they like to do so I take that into some consideration but it's all relative to the situation. Usually if I make it into a shop I have already looked at the tattooers stuff at least ten times on the internet and I'm ready to get tattooed. Plus If I am going to get tattooed by someone other than my regular group of tattooers its more going to be about what city I am in and who tattoos in that city.
I still look at portfolios when I get there because I always like to see the different ways someone puts one together and pass on the good ideas to the tattooers at my regular shop.