I may have inadvertently deleted a contact from my iMac

I may have inadvertently deleted a contact from my iMac contacts file. When I next sync my iPad/iPhone how do I bring up the dialogue box asking me whether I wish to change contacts on my iDevices? Currently this dialogue box only appears when the changes are the other way around, i.e. changes on either my iPhone or iPad; do I want to change the Contacts file on my iMac.

I think I really just need where to look. Thank you for any help received.