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The fact that you have no answer to this question means that your position is impractical. There is no purpose for a rule that cannot be enforced. What I would propose is this:

If you look White, act White, speak a European language, and fight for the preservation of the White race, then you are White.

You have so far offered absolutely no viable standard of your own or a technique for implementing it. This is because your position is motivated by divisive trolling rather than the preservation of racial purity.