I'm no printing expert, but I have a couple of thousand blotters in my collection, so I think I know a few details to look for.
1) The back of the blotters will typically be pink or blue, which means that there is paper that was glued or attached to the blotters. Others will have the printing directly on the blotter paper itself. Mostly white, but obviously the paper could be any color. I had one repop blotter which had green blotting paper.
2) A Texaco repop I bought, and thankfully was able to return, was on a single white piece of blotting paper. It was in almost too good of condition, and the colors were slightly washed out. The colors didn't look the same as the other Texaco blotters I have. In addition, the size was slightly off from all of the others. I don't know how it was printed on the paper, but I think it was a fake.
3) Another repop I bought had a printed piece of paper glued onton pink blotting paper. On ebay, it may have been tough to spot, but on closer inspection I noticed, again, it was in too good of shape, and wasn't the right size. When the paper started to peel in a corner, it was a dead giveaway.
I would compare anything you look at online to what you already own. They'll generally be the same size and look.
Feel free to message me if you would like to know what I have in my collection.
Collecting MN and Minneapolis gas and oil related items