When you do look in their portfolio, there are a few things to keep in mind. Do you see any photos of pieces that you recognize in the flash (on the wall, or in a flash book)? If so, how is it rendered in tattoo format? Before anything else, check to see that the lines are clean. Are they well-defined? Straight where they should be; not shaky or blurry? Are the borders all uniform in width? Do the colors seem true? Are they bright? Proportionately correct?
Look at the people in the book. This can be an indicator of the clientele in the shop (besides looking at the ambiance of the shop). Is there a fair mix of women and men in the book? Are they all sporting "biker" tats, or any one particular genre/style?
Again, keep in mind that anyone can stencil an outline of an illustration onto your skin. The skill in the artistry comes in the shading, use of colors and other subtle things that set an artist apart from a simple tattooist.
Do you see anything in the portfolio that is not in the flash? These are the custom pieces that the artists have done, and they should be their crowning glory. How do they look? Do you like what you see? If there is more than one artist working in the shop, and you see some photos you like, make sure to find out which artist did the work.