What are the surest signs that the wheel bearings are going out?

 

Rumbling sound I think?

Or jack up the car and rock the wheel in different directions and check for any free play (maybe in & out too). Just make sure any free play you find isn't actually your balljoints or steering tie rods moving because they're worn out.

Wheel bearing noise should increase with the speed of the car. Check first that it's not something rubbing (disc brake shields, etc.) Good luck.