Water pump, water flange on left side of head, hoses, radiator, exhaust. Of all the problems, the overflow tank ranks low, but easy enough to find. Just don't replace it 'because'.
A common leak Water flange cracks; water on the transmission... or the water pump is leaking/ not installed correctly. Rust from previous wp has to be cleaned out or the new gasket will ride up on the rust and if not immediately, it will leak sooner than it should. We do see some early failures on certain wp's.
We just lost a whole car to overheat... not my timing belt install... and the end result was a $4000 bill to recover. She opted to junk it. All because the mechanic installed without cleaning rust in the block bore for the pump.
Most common: The cylinder head water flange, water pump, cut or damaged water hose.
Visual inspection usually finds all of these. Take harmonic balancer and lower timing belt cover off to view bottom of water pump.
Troublesome: Head gasket blown from overheat.
Check for exhaust gasses in overflow tank. Bubbles in water return line are a bad sign.
Stranger places for leaks: EGR cooler, oil cooler.
Check with vacuum for leaks.