More than three in every 100 £1 coins is a fake, the Royal Mint said this week, suggesting there are now around 44m duds in circulation. But can you spot them, and what should you do if you find one?

The Mint says we should all be checking our change, and insisting on a replacement if we spot a fake. If you only spot the dud coin some time later, the Mint says you should take it to the police – that way it can spot any counterfeit hotspots. But it won't replace the coin – all you'll get is the satisfaction of playing a part in fighting crime.

Perhaps unsurprisingly, when we covered this on theguardian.com/money earlier this week, the Mint's advice got fairly short shrift. One reader even joked: "I can just picture the police station vending machine takings being 99% fake one pound coins."

Money tried to find out if anyone ever has handed in a coin to police, and the answers to a number of other currency-related questions posed by readers this week.

Is it true that you can tell a fake by rubbing it with a 1p or 2p?

The Royal Mint says: "Counterfeit coins vary in terms of sophistication so while this may be the case with some it may not be with others." There is a guide on the Mint's website (royalmint.com) to spotting a fake. Although the forgers' methods vary, there are a few common mistakes that should ring alarm bells. These are:

• The date and design on the reverse do not match (the reverse design is changed each year).

• The lettering or inscription on the edge of the coin does not correspond to the right year.

• The milled edge is poorly defined and the lettering is uneven in depth, spacing or is poorly formed.

• Where the coin should have been in circulation for some time, the colouring appears shiny and golden and the coin shows no sign of age.

• The orientation of the front and back designs is not in line.

Can you get into trouble for unwittingly spending a fake £1 coin?

It seems not. The Royal Mint says: "Under the Forgery and Counterfeiting Act 1981 it is an offence to knowingly pass on a counterfeit £1 coin." As long as you don't spend it "knowingly", you are in the clear.

Does anybody hand in fakes to police?

This is unclear. A spokesman for New Scotland Yard seemed surprised the Mint was advising people to take coins into stations, suggesting that if every coin was taken in officers would be kept fairly busy. In Manchester, the police service says the way crimes are recorded means it cannot tell how many cases like this have been reported, but that if a local area started to see lots of coins handed in, alarm bells would ring.

Has anyone ever been caught or jailed for making fake £1 coins?

There are no cases Money could uncover of people convicted for slipping a dodgy £1 into a vending machine or palming one off to their newsagent, but criminal gangs have been jailed for manufacturing fake coins. In 2010 a gang was jailed in Sheffield for counterfeiting millions of pounds of currency, including £1 coins. In 2009 a plot to bring in 500,000 fake coins from Italy was foiled, and in 2007 a man was jailed for forging 14m £1m coins in a workshop in Enfield, north London.

If you take a forgery to a bank will they give you a real one in return?

No. "Counterfeit £1 coins are not genuine currency and no value can therefore be given for them," says the Mint. Lloyds TSB says if you got the fake from one of its branches it will reimburse you, up to a limit of £10 – it will check that you are a customer and have made a withdrawal of coins first.

Do retailers get their money back if they find fakes?

It seems not. Sadly, Poundland was unable to comment on whether the number of fakes in circulation was causing it problems, so we contacted the British Retail Consortium. Its spokeswoman, Sarah Cordey, says counterfeit money is part of an increase in serious criminals targeting retailers. "It's not a new phenomenon though. Retailers have always trained their staff to watch out for fake money of all kinds and they remain on the alert. A big increase in counterfeit pound coins would be a concern but it's not something which is having a significant impact at the moment."

Are Manx, Jersey and Guernsey coins legal tender in the UK?

No. Royal Mint says: "The Channel Islands and the Isle of Man are crown dependencies of the United Kingdom. They have their own legislative and taxation systems and issue their own banknotes and coins. These, in common with United Kingdom coins, bear the portrait of Her Majesty the Queen. However, they are only legal tender within the crown dependencies themselves."

How about Gibraltar £1 coins?

The Royal Mint says responsibility for issuing coins in a UK overseas territory such as Gibraltar rests with that territory's government. It is customary for coins to bear the Queen's portrait and in some territories they are made to the same specification as UK coins.

However, says the Mint: "These coins sometimes find their way into circulation within the UK but they are not legal tender within this country."

Is it true that a lilangeni coin, the currency of Swaziland, is made by the Royal Mint to the same specifications as the £1 but worth 8p, and works in UK vending machines?

A reader says the face of King Mswati on the head side is a bit of a giveaway but the coin, which is worth 8p, does work in vending machines. The Royal Mint will not comment on this.