Anyone that is serious about their metal detecting should have an Acid Test Kit, and I personally can't imaging not having one. I can't tell you how many times I dug a ring or other item and I wasn't absolutely sure of the metal content.
Ever since I purchased one of theses Acid Test Kits that has not been a problem, and I have discovered a number of pieces of jewelry that I had were silver or of a certain karat of gold. You will find that many older pieces of jewelry don't have any markings, or a custom hand made piece might not be marked.
This kit is a must have for anyone swinging a metal detector and they are reasonably priced at Amazon. Here I have posted a nice kit that has the scratching board and a scale for a decent price. I also am posting an Acid Test Kit without the scale that I think is very reasonably priced that also includes an acid for platinum, and another that includes a diamond tester, scale and jeweler's loop, that is also very reasonably priced.
If you purchase a kit and are new at acid testing be sure to read the directions and the precautions very carefully.
Below is a picture of the results of a silver acid test that I conducted on 4 different items that I found while metal detecting. As you can see I scratched each item on the testing stone and then put a couple of drops of the silver acid on the scratchings.
What you need to look for with silver is that the scratching on the scratch test stone does not dissolve completely and turns a red or magenta color. The brighter the red it turns the more pure the silver. In my test below you can also see the bright reddish color that the acid test produced telling me that all 4 items were 925 quality silver.
The kits above contain 10k, 14k, 18k, and 22k gold, platinum and silver acids. With the gold test you should start with the acid strength that you think represents what your jewelry find is. So if you think a ring is 10k then start with 10k, if you think it is 14k then start with the 14k acid, etc.
If you scratch a 10k sample on the test stone then put a drop of 10k acid on it, there should be no reaction if it is 10k gold. If it dissolves then it is probably not gold. If it doesn't dissolve, then put a 14k drop on a new, or different part of the scratched gold, if it then dissolves it is 10k gold.
If it doesn't dissolve move up to the next acid (18k) and if it dissolves then, it is 14k gold and so on.
Remember to be careful using the acids and to carefully wipe or rinse the acid off. I also highly recommend wearing some rubber gloves while conducting any acid test. Remember to follow all the instructions carefully that come with the acid kits.