For the average US MD applicant aka No red flags, step 1 215-230 (I know I said average but really, just to match a serviceable program is plenty) this is my advice. All in all, manage expectations.
1. Programs to apply- Do about 30-40. You can definitely gauge the competitiveness of a program by just looking at their resident class list. If they have people from schools like yours, you are probably good to apply there. Vast majority of the programs out there. You'll get 10-20 interview invites with just applying like that. Go to about 10-15 interviews to truly get a good sense of what is out there.
2. Questions to ask/things to look for- Caveat of course still MS4 and no real knowledge other than all the programs i went to... But go to the dinners. Usually pretty fun to eat at nice places in different cities; if anything usually informative. Most of your true questions will be asked/answered at the dinner as that's when residents have the time. You can think of your own set of questions that are important to you. but here are some to get started:
-Basic stuff: call schedule, vacation, specialty rotations, attending/faculty relationships, things to do around the area/time off, how is the intern year (if categorical).
-Some specific things: quality of specialty rotations (when on CV/peds/neuro etc, are you only doing speciality cases or is it mixed in), autonomy things (how often inducing/extubating by yourself), numbers (what's one case/procedure/experience etc you wish you had more of), quality of lectures