New skylight positioning - how to avoid electrical wires

I am about to have some skylights installed (2x3 foot), in our late 50s flat roof Eichler.  Our roof is covered by several inches of foam roofing. 

How can we we be sure that there is no electrical wire over the spot where we are going to cut?   

Also, do we need to use redwood to build the box that the skylight sits on, or will any wood do?

Thanks,

Steve