Going the other way, I've had a fair number of intact male dogs come on to me at dog shows and the like. More than once, I've even had a dog stand up with his paws on my shoulders and his sheath opening right in front of my face. Of course, it could just be my imagination running away with me, but maybe not. I find myself wondering, "how can they tell?" Reading this thread, now I know. More likely than not, I'd had my dog's penis in my mouth that morning or the previous night. The dog I just met can smell that and could be wanting to try his luck with me.
Having chatted online with more than one show handler who engaged in various sexual behaviors with the dogs, it wouldn't surprise me. One talked about reserving it as a reward for an outstanding performance.
Some years back, I visited a State fair where the Royal Canadian Mounted Police were doing a demonstration and they also joined the fair's parade.
While wandering through the fairgrounds' stables later, I noticed one one of the Mounties (out of uniform) in one of the stalls with a smallish, black mare. He was kissing and stroking her and talking softly to her. I said, "Sweet little mare!" He replied, "Yes, she is." and shyly turned away. Since no one else was around, I wanted to say something like, "Dude, I totally understand, You're among friends here." I decided it would be a really bad plan and kept quiet.