Had a call last winter where the homeowner handed me a receipt from a sweep dated six weeks before. Pulled the cap and the flue was caked solid, no way a brush had touched it. Turns out the "company" was a guy with a shop vac and a flashlight charging 150 bucks cash. Now I tell folks to watch the sweep do it or get the photo like you said, and I also tell them to stick a mirror up there themselves once a month during burn season. A cheap handheld mirror and a flashlight will tell you more than any receipt ever will.