I think the key is to be realistic. If you're brand new on a bike, spend several months getting more and more comfortable on quiet side streets and paths before you start taking to crowded streets and roads. If you're new to biking with heavy panniers or towing a trailer, you want to be very comfortable handling a bike (stopping/balancing/standing on the pedals/etc.) BEFORE you're biking back 8 miles with 60 lbs of groceries . . . or a kid.
Some basic skills that you should use to gauge what your skill level is:
- Able to balance on a bike without problems.
- Able to stop a bike quickly using the front brakes (you have to lean back), the rear brakes (you have to learn to control the skid), and both. Also, you want to learn how the brakes stop you differently depending on road conditions.
- Able to look over your shoulder while biking in a straight line.
- Able to keep the bike going in a straight line with only one hand on the bar (required for signalling)
- Comfortable balancing on a bike while standing and hammering the pedals (required sometimes when climbing and carrying lots of weight).
- Comfortable biking around cars (this is mostly mental, but I know many who are afraid of biking on roads).
- Comfortable biking in bad weather (high winds, snow, on dirt, potholes, etc).
- Comfortable descending hills at good speed.
Once you're cool with the above, you should be good for just about anything that you encounter on a bike.