EndofFashion - I am so glad that you have come to this forum to ask this very good question. I deal with about 8 rescues in Ontario, 2 in Alberta, and 2 in Quebec.
Reputable rescues should be able to provide you with the following:
Approximate age, breed, past history, a disclosure of all medical history if possible, medical status now, any quirks, a STERILIZED and fully vaccinated dog or at the least a follow up on sterilization at due date, a health guarantee of at least 30 days. You ask for copies of ALL medical documentation.
If in foster, you should have one on one contact with the family to ask all the questions that you want to know.
Also, a rescue that puts a dog in a 'foster with intent to adopt' is ideal. This gives you a great opportunity to ensure that you and your new companion mesh well before making that commitment.
Not that I am the all be all, but I do know a few really great rescues that can help you. I've been doing this for years.
If Marco will give the permission, I can provide you a list of very good rescues that will help you and you will feel confident about the service you are getting. There are no hidden facts or agendas with the rescues that I heavily rely on.
If you need help - I will be more than happy to assist and put you in the right direction.