It is difficult to tell some of the fake strip cards on a computer screen. For higher end ones I would suggest buying graded if you aren't super experienced with them (most of the times they are good if graded,) and/or from a reputable person. There are lots of fakes....
To answer the question, personally, I look for fuzzier images and lithography on fakes than on real ones. The image and lines are not as clear on most bad ones.
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