How do they know?

We were up at the allotment briefly this morning and chatted to some of the old boys, after harvesting the rhubarb and getting some chives for lunch.
"It'll rain later" says one,
"How do you know said OH-what signs are you looking for?" (hoping for some old country saying I think)
"I listened to the weather forecast!" was the reply.
"Ah!" says OH