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The way I did it was to seat the rings one by one in both the block and the manifold, and then use calipers to measure the amount of ring protruding from the surface at the highest point for each (because on the block side it's unlikely the rings will seat perfectly flat).
You write it all down and then you pair up the numbers for each exhaust port on the block and the manifold. If the exposed ring width on cylinder #1 block-side plus #1 manifold-side is less than or equal to the total width of the ring, then you have sufficient clearance on that cylinder. If it's barely greater than, you should assess the expected width of the compressed gasket and make the call.
Side note, Remflex makes exhaust manifold gaskets that are extra-puffy, which helps with sealing and gives a bit of additional clearance, if you don't mind the non-copper look.