How do tell what psu my pc is compatible with?
You don't have to worry about psu compatibility. What you have to worry about is if it will fit in the slot and if it has the proper cables; i.e molex cables if the gpu requires them. A psu is one of the only pieces of hardware that doesn't have to match the motherboard/cpu.
not necessarily true. Some manufacturers use proprietary PSUs. DELL is famous for this. Most computer makers, however, use a standard PSU format.