All official Fumos originally came with either a tag or a tin badge. If that is missing, that could be a sign of a bootleg (although it's possible the seller lost the tag or badge, or bought it from someone else who originally lost the tag or badge). A missing tag will generally lower the price too.
Keep in mind that Fumos originally sell for around $40 (or around $500 for a Deka). If you see a price lower than that, it's very likely to be a bootleg or other fraudulent listing.
Look at the photos in the listing. Try reverse image searching them to make sure they're unique. If the Fumo looks particularly derpy, it's probably a bootleg. You can compare against the stock photos in our Fumo List.
There are some bootleg Deka Fumos with very long legs (the originals have the same stubby legs as their smaller counterparts).
This is a list of sites used by independent Fumo sellers.
This is a list of common and/or popular online stores that often have Fumos in stock.