if you turn the bike upside down, and spin the wheel, look for "bounce" or high spots. You can use a marker to mark where they are. Then look where the tire is seated on the rim, where it meets the rim, and see if it's even all the way around.

And yes, the wheel bearings aren't very good, but they are as good as the other components I guess.

Anything you do on a sidewalk is dangerous in my eyes, especially in a residential area. people pull out of driveways not expecting someone to be moving at 20 MPH on the sidewalk. People take turns with the same lack of caution. You are in more danger on the sidewalk that on the road. AS far as the bike goes in this situation, if everything is up tight you should be fine. It's the other aspects of being on a sidewalk that are dangerous.