When you're out somewhere buying bullion and don't have access to a XRF or Sigma, there's a couple quick ways to test for fakes, so you don't get scammed.
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Magnets
Gold and Silver are diamagnetic, which means a magnet won't stock to them, but a strong enough magnetic force will cause them to move. Your average fridge magnet isn't strong enough to do this, so you'll need a rare earth magnet (sometimes called a Neodymium magnet). Fortunately, you can get a portable magnet relatively cheap on Amazon, for around $6 or less.
Not only should the coin or bar not stick, but it should move slightly. If it doesn't move, it could be a fake made of a non-magnetic metal. If you're still not certain, there's another test you can run.
Pocket Scales
If you're buying bullion or precious metals with a known weight, then you can easily weigh it for accuracy. If it's supposed to be an ounce, it should weigh 31.1 grams or more. The same goes for larger pieces of bullion. If the weight is under, then you're likely looking at a fake. Scales are a light and inexpensive way you can test bullion. At around $15 or less, it's a worthwhile investment to prevent being scammed.
Calipers
If the piece you're considering buying is more common and has known dimensions, you can also test for fakes using a set of calipers. If the item isn't made of pure gold or silver, then the dimensions will be off in either the thickness, diameter or both. These are inexpensive and usually cost around $10-15.
The "Ping" Test
Silver and Gold coins have a unique ring to them when you put them on your fingertip and tap on the edge. For larger coins this is relatively, but it becomes more challenging when attempting this on smaller coins under half an ounce in weight. To counter this issue, one company has made a device called The Pocket Pinger, which holds the coin for you so all you have to do is pull down and let it strike a plastic ledge to create the ping sound. This device typically costs around $25.
Alternatively, you can also simply hold larger coins on your fingertip and give them a light tap with a wood dowel. This way you get the ping sound and won't damage the coin or round. You can find these relatively cheap on Amazon.
If you're new to stacking and/or this type of test, you can also download coin ping apps for iPhone and Android, which use your phone's microphone to measure the sound waves. It then compares them to those of a genuine coin to determine if yours is real or fake. These apps typically only have samples for popular coins, which is why the other tests are equally important.
While each of these tools individually won't completely weed out fakes, combined they will help prevent you from buying fake bullion without having to buy or borrow expensive testing equipment. Remember that as a general rule of thumb, if the deal seems too good to be true, then it likely is.