How do you know when you need to replace the front wheel bearings?

You'll know by the horrible droning sound being made when you are driving. I thought maybe the noise was coming from the tyres as they were the only things to have changed. Once I got past 30mph the noise started to get annoying. I started looking into tyre reviews. However, after a 200+ journey the noise was too much and decided to take it to a garage and they said there was too much play in the wheel and unsafe to drive back. It turned out to be the rear passenger wheel bearing being the fault. Had it changed, cost around 170GBP and haven't a noise like it since.