Early spuds - how can you tell when ready?

Planted them in the polytunnel in January under some bubblewrap. They are about a foot high now. When/how can you tell they are ready?

 

If they flower they are probably ready, not all spuds flower.

Weeks-from-sowing is a good indicator,. but for a January sowing that may not be all that reliable as the first couple of months will have been cold and with little light.

Have a "furtle" in the soil and see what's there is the other option. I find that tends to kill the plant though ... maybe I'm just hamfisted