The content of these vary enormously. Send off for one or two, particularly if their adverts sound good, and use them as a benchmark. A good profile will include most of the information you need. If there is information missing, phone the practice manager and ask. Be prepared to be put through to a partner so don't call about trivial points and avoid traditional surgery hours.
First impressions count.
Ask locally about the practice. Other practices that you work for are likely to know the partners there and receptionists will know other practices' reputations - they may be registered there themselves. Ex-registrars can be very useful although their views may be coloured by the way they were treated. An informal look around the building is essential if it's not too far away. Practices should be prepared to do this.