Q: how do i find the apple id i set up my ipad with

Hi,

I've recently had problems with my iphone which has been reset to factory settings. But now my ipad which has the same apple id has a problem.

It keeps telling me i'm using the wrong apple id and that i need the one it was set up on even though i thought it was the same one as i have now.

How can i find the apple id i used for the original set up?

Cheers for any help.