Sometimes it’s obvious who someone is voting for. When I was a Virginia voter in 2012, there were Republican and Democratic volunteers at the poll stations who would hand out voting guides to each voter - pink for (R), blue for (D.) Usually, people would only take the booklet for their side and decline the other’s. You could generally tell, just by which voters were holding which color of booklet, which side they were voting for.
I made a practice of holding both in my hands, so people couldn’t tell.