If you are thinking about Radde's, which is the most frequently occurring similar species, the supercilium is a good pointer, being well-defined in front of the eye, and becoming diffuse behind the eye, while it's the opposite in Radde's.
I guess there's heavy passage of Duskies at the moment, as whenever there is, one pops up in the garden, usually only on one date in mid>late October, and we had one yesterday (30/10).
Mike Turnbull.
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