There is no single magic bullet "tell," but here are a few red flags to look for.
Sticker quality appears different. Poor print quality... different sheen, etc.
High value item that only has a sticker... no card or full letter. Bear in mind items valued over a certain amount MUST have a letter.
Number does not match item description on website
Fake stickers tend to appear on items that may appeal to casual / impulse collectors. For example, chances are you're not going to see a fake sticker on a Thomas Edision letter, but more likely items like LeBron James, Michael Jordan, hot sports and entertainment celebs, etc. In my experience, most fake stickers I have seen have been on sports items.
Fake stickers tend to come from no name eBay sellers who offer other suspect material. I really don't recall ever seeing a fake sticker offered by a known and reputable dealer.
Also, bear in mind that fake stickers are uncommon and represent a tiny fraction of the overall market. Don't drive yourself crazy obsessing over it.