Earlies will as the name suggests usually flower first but this is not always the case as some varieties do not flower. The best way to tell, is to gently push the compost aside without detaching the tubers from the roots so if you pick the wrong ones first, you can just re-top up the compost.

Also you might notice that the tops of main crop continue to look good for longer than the earlies as there is a tendency for them to start looking tired and dying back when the potatoes are ready.