Some years back I was zeroing a thermal scope, and I figured out I needed a 3 inch offset at 25 yards. This was the zero necessary for the rifle due to the massive height over bore with the thermal optic. Unfortunately, when I would look at a target though the thermal scope, the whole paper target would be the same temperature, and I wouldn’t be able to tell where to aim.
What I did was tape a quarter to the paper. The metal of the quarter was a very noticeable different temperate than the target paper, and that allowed me to zero. Once I was make the appropriate adjustments. I moved the quarter to a fresh part of the target and set up a Shoot-N-C patch below it to check this zero.
As you can see above, I was impacting where I wanted to be. If I recall correctly, that was a 3 shot group.
Now some thermal scopes are sensitive enough and high enough resolution that you can tell the markings on the paper due to the darker areas soaking up more heat. But if you can not, you can easily do something like tape or glue aluminum foil, or other materials to a target to make it work better with thermal.