None in most cases (that is how jewellers pass of 12 carat gold as 18 carat and make more money. If you have an invoice stating that it is 18 carat, you can take it to another jeweller and pay him to have it professionally valued and then if it turns out to be less, you can get your money back.

Currently it will just include the number 750 in the hallmark. Slightly older items will have the 750 next to a crown, and anything before 1974 will have a number 18 next to a crown. Those numbers will appear in a rectangle with the corners cut off (that shows it is gold). Numbers in an oval are for silver, and in a truncated cone for platinum.

If it isn't from the UK - it might not have any marks at all, or could have all sorts of weird and wonderful things.

The markings mean nothing in this day and age. Most jewelry is made in China or India. Once it leaves the factory they take on responsibility for it, and nobody will hold them accountable. I8K gold is very often 14K gold. If you bought it online then it is more than likely only 10K.

Also it may be plated with 18K gold.

Look for 750 in the hallmark.

It should have proper hallmark as well as indication of purity (makers mark, date etc.). If not it is just plated.