Con men cannot produce a real naira note. That's a fact. Even if they can, they can't produce real note without bloating overhead cost. In view of this, there is always obvious differences between a real naira note and it's counterfeit. These differences are not hidden; they are there only if you look well enough, even for an untrained eye. You should also avail yourself of the fact that the use of counterfeits is punishable under the Nigerian constitution whether you're aware or note because "ignorance is not an excuse in the court of law".

Taking advantage of the marked differences between real and counterfeit naira notes, I'm going to list ways you can identity counterfeits\1. Through Mercury Bulbs

In the real paper naira notes (?100, ?200, ?500, ?1000), they are some texts that are not visible to the Unclad eye; they are only visible through the rays from a mercury bulb. So, to verify the authenticity of a naira note, bring the said note in contact with rays from a mercury bulb. If it's real, you'll see a greenish-yellow glow of the note's denomination across it. For example if it's a ?1000 note, you'll see a glowing 1000 (in numbers) written across the note and smaller 1000 written on specific spots on it. The same goes for other paper denominations.

If the money is in a stack or bundle and you want to test for counterfeits, arrange the monies (it should be the same denomination all-through) properly (i.e. the front of each note in the bundle in contact with the back of the next note and top to top and bottom to bottom) and subject a side of it to rays from the mercury lamp, the greenish-yellow glow should be visible on the first and last note in the bundle if no counterfeit(s) is hiding in it. In either the single note and the bundle (if properly arranged), absence of this greenish-yellow glow means the note or a note in the bundle is fake.

Sorry, I couldn't get you the picture but the glow is very visible. Make sure to switch-off other light sources so as not to hamper the result. While mercury bulb is available at shops where electrical materials are sold, this method is preferable for business owners or people who handle bulk cash.

2. Through Water Or Other Liquids

The colours used in printing counterfeits are soluble in water and some other liquids while that of real money is not. Wet the suspected money or a part of it with water or any other liquid—I have only tried water and petrol—and scrub the wet part with your thumb. Counterfeits will wash-off their colours as you do this but real will not. Do you notice the way the colours of an artwork painted with water-colours wash-off when water touches it? That's the kind of wash-off I mean.

3. Through The Ribbons

On every paper money on the naira is a thin silvery ribbon running from the top to the bottom of the note; it's roppy on old notes.

In real note, you can feel and even pull-out this ribbon on some old notes.
However, in counterfeits, there's something that looks like a ribbon but it's not—just paint. Try scratching that ribbon, it come off like the silver panel on a recharge card.

4. Paper And Colour Quality

While counterfeits are made of ordinary papers, real money is made of a special kind of paper. Fealing the paper-quality of counterfeits, you'll find out that it's just like that of paper found on the street.
The colours of counterfeits also betrays it. The drawings on counterfeits are more blurry, blotchy and sometimes more darker than real paper money.

Of the four methods listed above, the first, second and third is more reliable.

Source: http://nignews.com.ng/how-to-identify-fake-naira-notes/