How do you set timing on 3.4 liter Chevy engine after the timing belt has broke?

Assuming you're talking about the TDC (dohc) motor. Unless you are Very familier with this engine I would'nt advise you to try setting it on your own. The 3.4L dohc or TDC engine requires very precise cam timming. There is only a .010 inch valve clearance between the valves and piston. In most cases, when the timing belt breaks on these engines the piston strikes the valves. These engines have sodium filled valves and are easily damaged. You should consider having a compression test done before even attempting to install a new timing belt. Which also should be installed by a qualified technition. The timing marks are "painted" on @ the factory and probably are no loner visible.