veggie burgers

Question

rabbi you said that you can eat a veggie burger in Burger King if its 100% VEGETARIAN - provided its in a seprate microwave ... how do you know they didn't use the microwave for other non kosher foods? also how about BISHUL GOYIM ? MAARIT AYIN? and if you didn't see them open the box in front of you how can you be sure? i don't trust some goy behind the counter telling me its vegatarian what does he know? also what about the french fries ? looking foward to your reply