Trolling the internet? I hope you meant Trawling. The nominal transponder edges are defined in the config files where you set up each satellite. The actual centre frequency is not a parameter but you can work it out from upper and lower. To find it during a pass you will need to compensate for Doppler. The offset is displayed in real time.
Unfortunately, not all amateur satellites carry frequency standards and drift quite a bit with temperature. Consequently it can be really difficult to know where the centre is.
ERP - None at all until you receive the beacons, then the minimum to hear yourself come back, but never ever more than the beacon. In practice a watt or two to a small yagi is sufficient.