The piston goes up and down in the barrel and every time it's as high as it will go that's tdc. But the rocker arms can only be free play wiggled every second time the piston comes to tdc. You'll need to work out which of the two tdc's is which.
Ted route is the way to go. Take the plug out and poke something thin down the plug hole. Turn the engine over slowly - otherwise the screwdriver or whatever you're using can get jammed in place, and feel the piston come up. When it gets to the top (you don't need to be exact - near enough is good enough) check for wiggle. If the valves are held down or moving you need to turn the engine over once to the next tdc and check again.
A quick way of doing it is to take the plug out and put your finger loosely over the plug hole. Turn the engine over and you'll feel air escaping past it on the compression stroke. That's the correct tdc.
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