Well this largely depends on what currency you're talking about, in America and most places a special type of printing paper is used to make the dollar bills but in Nigeria some notes are made from paper and others are made from a plastic like material.
Seeing as paper notes are the most common in the world I think I'll just identify a couple of things that you should look out for when you suspect that money is fake;
1. Check the texture: Always feel the note first, if it feels too soft or furry then please reject it. I once saw a 200naira note that looked a lot different from the other ones I'd seen, once I held it in my hand and felt the texture I know it was fake and I immediately demanded another one.
2. Water test: If it's a fake then chances are the materials they use are going to be cheap as well. If you suspect money of being fake, dab a little water on it and watch and see if the colours run off, if they do then please return the money and demand for a new one immediately.
3. Check Unique markers: Most currencies have something about them that can't be imitated easily, with the dollar it's the colour shift on the bottom right corner where the value of the not is written, on fake notes there won't be a colour shift. In naira notes, there's a reflective transparent strip which passed through the center of the note that can actually be felt, on fake notes it'll just be a painted on strip. If you have any currency and they have unique markers then check and see if the ones on the note you suspect to be fake are there, if not then return and collect another note.
4. Check the colour: Sometimes fake notes may look a little darker or lighter than normal notes so watch out for notes that may seem this way. Hold it up beside a note of proven integrity and confirm whether or not the note is fake.
5. Use mercury bulbs: Some countries have hidden features on their money, some of the naira notes for instance have writing on them that you can only see under a mercury bulb, if you're convinced that the money you have is real or fake then this is one sure fire way to test it.
No matter the currency, if you check all these things and the note doesn't fail any of these tests then it's actually legit, but if it fails one, then it's fake!!
I hope this helps.
It is easy to spot along with touch.
Paper money is made using a special paper made out of "paper" which is 75% cotton and 25% linen. The reason behind this is when you wet a bank note it wont fall apart and the ink wont disappear.
Using this special paper makes it harder to fake and one should feel a significant difference from a real note to a fake.
The markings on the note itself. A special watermark has been imprinted into the note and this can't be replicated. If you hold the note up to the light you will see the watermark. If you can't see this feature the note is fake.
The latest notes have a distinct pattern using yellow dots that can't be copied by printers either.
If you know what a genuine note looks and feels like you should never fall fro counterfeit money o matter how good the copies are.