I would imagine if there was any pre-wiring to the roof, there would be some kind of cable cap on the roof visible?
For cabling, I'd probably go for 4mm cable tops unless you are installing a lot of PV wattage. If you were to install something like a Victron 100/20 (good for upto 290W) or a Victron 75/15 (good for upto 220W), you will find the 6mm Cable a right faff to fit. 4mm cable is fine for just about any array you are likely to be able to fit on your roof.