If you're unsure, one of the best ways to determine a coin's authenticity is by weight. All U.S. coins have a specific weight, so a real one will be correct, and fake will weigh something different. If the coin is heavily worn, it's possible that it could be slightly underweight, but not by more than a couple percent.
Yes. An authentic 1862 $5.00 U.S. coin is a gold coin.
The best thing to do is take it to a coin dealer.
This coin has a mintage of 47,000. To be certain the coin you have is authentic, the best thing to do is to send it to a well known grading company and have it certified. This will tell you if it is genuine and what its grade is.
One method to determine the authenticity is to weigh the coin. A genuine U.S. silver dollar should weigh 26.73 grams.
Unless the cashier is a coin collector, probably not. They won't know whether the coin is really gold or a replica, and they won't know if it is legal tender, and likely will not accept it. It wouldn't be a good idea to spend gold on gas anyway, really. You could get much more than face value for an authentic $50 gold coin.
If you are talking about an oversized coin as in a large replica, the answer will most likely be no one. Unless your coin is an authentic coin, it has no collector demand and you'd be best off to sell it on eBay or a garage sale as a curiosity. Naturally, a coin dealer will buy authentic coins, but will not buy replicas.
Take it to a reputable coin dealer who should be able to tell you if you have an authentic pattern coin. Most pattern coins are not authentic and therefore numismatically worthless. But if you have a real one it can be worth quite a bit.
In general there is no such thing as an "authentic" two-headed coin. Since "Heads" and "Tails" are stamped at the same time on a stamping press that has different dies for the top and bottom, it's almost certainly a "manufactured" coin. If it's done right, it can be difficult to detect the seam.
7-1-11>>> This is not a US Mint error coin. It's a novelty or magician's coin with no numismatic value.
Please look at the coin again. No authentic 1816 U.S. one dollar coins exist.
Most likely it is real. However to be sure just take it to your local coin collector.
The piece is not authentic, but a coin dealer may be interested in it.