I am thinking about getting http://kopes.com/gadgets/invisible-ink-pen-uv-light.htm mark sheets, towels, pillow cases. I have been using a pencil and marking the seams of these. Guess what. They are still there when I come back. I have called the 1-800 not happy # for the hotel. I have found that its actually breaking the law in several states to not change the sheets.
"3731.12 Beds and bedding.
(A) Every transient hotel and extended stay hotel shall provide in each sleeping room or extended stay temporary residence a bed, bunk, cot, or other furniture designed for sleeping for each guest occupying such accommodations. Every hotel shall provide each bed, bunk, cot, or other sleeping place for the use of transient or extended stay guests with pillow slips and under and top sheets. All sheets and pillow slips used on any furniture designed for sleeping shall be white or off-white in color and shall be washed daily if requested by a guest, and all such sheets and pillow slips, after being used by one guest, shall be washed before being used by another guest."