E30 320 ignition timing

I have a 1987 320i with an ignition timing problem. I initially thought that its timing was 100% electronic, but it can be advanced/retarded by turning the distributor. The problem is that it PINGs often when cold. It
also sometimes pre-ignites while you are trying to start it. It would sound like a dead battery and then it will start swinging normally.
How do I check the timing to be in the right position? As far as I know, there is no mark on the pulley.
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