if you see someone wearing a beautiful sheitel, ask them where they got it and how happy they are with it, that is best way. you have to like the appearance, and you can be the only judge. asking strangers? who says you like what they wear?
Not all Shevys are good quality. Ask, ask ask.
One more thing: ask the person if she needed any service after the purchase, how easy it was to get to the seller. A Kalla friend bought a new expensive one from someone who sold her a nice shaitel, and did a fine job styling, and promised to be available during sheva brachos for touch ups. it has been 2 yrs and she still didnt return any of the calls and msgs with kids etc. Collecting 3.5k was quick, anything after was impossible. so ask in advance.