Yup.
-Look at the label, are there two small numbers engraved on it? Is it glossy? If not, likely fake. The label not being well positioned is a good, but not perfect, indication too. (for GBA I mean)
-Shopping on Ebay, is the price too good? Is it shipped from China? If yes, likely fake.
-Ultimately, buying a 1$ screwdriver is the best way to spot a fake as well. Once you compare a repro board to an authentic one, you can make the distinction easily. But the two options above are good steps to do when shopping as well to avoid some unwanted frustrations.
In my ten years of collecting I ended up buying a fake once, I promptly returned it and got a refund. Ebay will take your side, especially if it's not written anywhere in the item description that it's a repro. Surely it won't be as easy today as back then, but heh, I see this as some little extra challenges.