Base metal that resembles gold the most is brass, although brass isn't technically a metal, it's an alloy made of zinc and copper. These two metals are used because they have color which is very similar to that of gold. Various ratios of brass and zinc are used in order to create different colors, depending on the taste. Even though detecting brass is relatively easy, when compared to other methods of creating fake gold, there are still a lot of people that do not know even the simplest of tests are used for detecting real gold, so they can still easily be scammed.
The number one thing which can and should be used for detecting brass would be the size of the item that you're checking. Remember that larger pieces of jewelery or some other kind of larger ornaments made from pure gold are almost never large in size. The reason for this would be the fact that gold is very expensive, and normally you'll never come across such large items, up to a few pounds of weight, made entirely out of pure gold, or even gold of lower quality for that matter because it would cost enormous sums of money if it were created, and enormous sums of money isn't something that a lot of people can afford, so there's no point in making items such as these. If you do come across something of that size, you can rest assured that it's brass, which is considerably less cheaper than gold. Usually whenever someone tries to sell large brass ornaments under the guise that it's gold, they are selling them for a lot less money than what the market price of gold is since even at that decreased price they would still be making quite a profit. This would mean that low price can also be used as a tool in brass detection.
Moving on to tests that are a little bit more conclusive and which require tools. We'll start with the easier one first, testing with a magnet. For this test you're obviously gonna be needing a magnet. Magnet is using a difference in physical properties of brass and gold. Brass isn't magnetic, but it's corrosive in nitric solutions. Acids are the thing to use in order to be sure, since brass, plain brass cannot fool the acid test. In order to check for brass with acids, you first need to get your hands on an acids, nitric and muriatic acids are very popular for doing this, just pour a drop or two on the piece, preferably somewhere where the damage, if it occurs of course, won't be visible so that even if it's not real gold the item can still be used by someone. If color that occurs on the spot where the acid was changes to black, then you're dealing with brass or some other alloy that's been made to look like gold.
These tests should be more than enough to detect brass, but do remember that this is only if the plain brass is being planted as real gold, if for example brass is heavily plated, it won't react to acids, but it should still fall on the magnet test. Magnet that you should use can come from pretty much anywhere, old speakers that no longer work for example. When it comes to acids you might have problems if you try to receive them via mail, because some states do not allow their transport that way. Last piece of advice for detecting brass would be to carefully examine the item, sometimes it's not necessary to go through the tests, because brass is marked, similarly to gold, with words "brass" stamped somewhere. Take all the necessary precautions when dealing with acids, do not allow it to come in contact with your skin, use gloves, safety first.