I usually know, but the last time I did not and neither did she. We live in a small town area (only 12k people in our entire county) so it isn't that uncommon to run across a guy she has fucked while out and about. She has this crazy way of running across a guy to fuck at Walmart, of all places.
Saturday she came back from getting groceries, came through the door and told me I needed to go carry them in, then said "But first", stepped out of her sweatpants and sat back on the couch with her legs spread, which was my cue to get busy cleaning. No complaints.