I think I mostly saw people grab it by the seat and bars.
however I did once sell a bike to a woman who never testrode. she just stood beside the bike and grabbed it by the stem and bars as if she was walking it. she said this is ok and bought it!
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk