Open the record cover (record removed), so it is shaped like so: ()
Stick your hand in, with a short marker (small sharpie marker) and put your name or some identifying mark. Make it so it is at least half way into the record cover, so that someone really, really has to be searching for these marks, in order to find them.
Alternatively, a box of small printable labels from the local paper shop. Put them in, all the way to the back. This may catch on the record so go for putting them in the back corners, where the record will never approach.
I used to do ’sneaky marking’ of my textbooks for college/university, as the prices were so high, they’d get stolen.
Imagine the mayhem when I catch the guy with my stolen textbook, right in the class. Happened more than once.
In reality, I kept it calm and cool, as we’re looking at a 2-3-4 year run with the same folks in the classes. Nobody’s got time for anger and angst that lasts that long. Take the high road. Of course they needed the textbook as much as I did. So we photocopied sections as the classes moved along through the text. When enough people dropped out, they got the text for cheap. Fixed, no fuss. Might even make a friend.
I'd forgotten about all that stuff way back when.
However, I can sympathize abut the records. One time I had a brand new stereo stolen (break in) the day after I bought it. I had no money, going to school working full time..scraped that money together to buy some separates....and..well...