You won't find a physical "number 1" marking on the distributor itself. You have to use the engine's timing marks and the firing order to determine it. The firing order for a typical 350 small-block Chevy is 1-8-4-3-6-5-7-2. You'll need a timing light or a dial-back timing gauge to find the exact TDC and align it with the rotor pointing to the #1 spark plug wire terminal.