First thing is to be wary of prices you see on the street or on marketplaces without reputation. Original Funko Inc. Pops have a pretty stable price range depending on whether they're exclusives or limited editions, but nothing should be dirt cheap. Big online stores with good reputation (like official Amazon, Fnac, or sites specialized in collectibles) usually guarantee authenticity because they have more to lose selling counterfeits.
A trick that works: before buying anywhere, check the seller's profile and look for reviews specifically about Pops, look at photos from other buyers where you can see the box and the product. On social media there are collector communities where people share comparative photos of originals vs. fakes; they're super useful for training your eye. The details that most give away a counterfeit are the print quality on the box (blurry letters, dull colors), cheap plastic on the figure, and details like the eyes or uneven paint.
Also, if you go to physical stores or local marketplaces, ask to see the box in real conditions, touch it, check if the barcode is sharp and if the box has that finish that a legitimate Pop has. And if you've already bought something that seems suspicious to you, many big platforms let you return it no problem.