Max RPM can usually be determined by the engine not winding any further (usually the intake/exhaust reach it's max capacity or the valves float) or the engine goes BOOM usually followed by expelling pieces and fluids in various directions.

On non hi preformance american V8s (cast crank, 2bolt main, stock rods, stock valve train) 5500-5800 a safe realistic red line.

In my mis-spent youth I had a "built" 289 in a 63 Falcon Futura. Shift point was 7200. It lasted about 5000 miles before it spun 2 bearings.