For example, how can I ask to rate my English skill the way it was 6 months ago.
I'm almost sure it's wrong but something like:
How would you rate my English at 6 months ago?
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There are many different ways to say what you want to say with that sentence. Here are just two of them:
On a scale from one to ten, how would you evaluate my English speaking skills as they were six months ago?
On a scale from one to ten, how would you evaluate my English as it was six months ago?
Your original sentence might work. The only serious problem that I see with it is the phrase at 6 months ago. That just doesn't sound like natural English in the context of your example.