When are peas ready for harvest?

Dear gardeners,

I planted peas in march and doing well and a lot of pods are forming but when do you know when to harvest them? Last year I took of swollen pods but contained very small peas so clearly weren’t ready for harvesting. Is there a trick to determine peas are good and ready?

Your advice is much appreciated.

Pick a pod and eat the peas 

I find they rattle a little bit if you nudge them on the plant when they are ready.  But eating them on the plot is my favourite way