I always make sure to ask if they can write me a strong LOR rather than just an LOR. It probably doesn't really do much, but it does allow the prof/doc to question themselves as to whether they could actually do so. I also look for enthusiasm too. I asked an MD that I shadowed, and she was just like, "Okay...", and I kept having to remind her about the letter. I ended up seeing it because she handed it to me to read, and it was overall positive, but I didn't feel that it was very strong or personal. Needless to say, I didn't use it.
I had one professor ask me to meet with him after I asked for an LOR, and we spent an hour talking about my motivations to be a doctor, etc. I also used this letter to apply to an SMP program. I'm now working in the program office and one of my colleagues has access to the letters we used to apply, so I asked him to read it over for me and tell me if it was good or not since I'm reusing the letter for this application cycle. He said it was really great! So, I guess whenever a prof/doc is enthusiastic and/or asks to meet with you, then you know you're probably going to get a good letter. Of course, this isn't a guarantee.