You would probably get some spuds now - depends how big you want them! If they are earthed up you can scrape back a little soil to expose one so you can judge whether you think its ready. You can reckon 12 weeks from planting as a rough guide for earlies so its well worth checking.

Usually after flowering when the above-ground growth dies back. If the soil's loose enough to have a feel around that'll help you get an idea of what's happening but make sure they're well covered afterwards or they'll turn green

The flowers signal that the spuds are almost ready. When they fade completely, then go to it.

Thanks guys, I learned something from those replies too.