how do i know if my iphone is unlocked

hi all, i am new here and new to using an iphone, i bought a iphone 3gs on ebay which works well on EE network, we live in a rural area where there are some network signal issues, my question is there a way of finding out if the phone is unlocked to any network without putting diferent sim cards in? i have a giffgaff card which runs off O2 network but it doesn't seem to work, yes it is activated,

any help with this would be good thanks Petal