i think my rad cap is knackered but how can i tell. sorry for the stupid question

 

I has similar concerns with my uprated Cusco, check seal and check spring by putting pressure on it to squeeze...it needs a decent bit of force to push down.

I also found out (cheers Zen) that the cap is designed to suck overflow fluid back into the header tank, its got the funky valve in the middle that can be pulled open allow fluid to flow back.