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The way I usaully can tell if it's a butterfly or a moth is:
A butterfly has the bulb on the ends of the antennae, and the moth has feathery things on the antennae. Plus, there's a 'very' different look of the wings.
And there's probably other ways to tell.
Ulrich, there are also several diurnal moth species e.g. Hummingbird Clearwing Moth.
The thickened end on the antenna as Xenomorf mentioned above is most likely the best way to distinguish. Not all moth have the feathery antennas and some have filamentous antennas but lack the thickened ends.