Google "test pool salinity". A lot of swimming pools use salt to produce chlorine. $5 - $10.
KH test.If you search "pond ph buffering" you can find out why if you understand KH and buffering you don't have to test pH.
Ammonia test. Fish pee produces ammonia and is harmful. Bacteria convert the ammonia into other stuff making it safe. Ammonia should always be zero or very close in a stable pond. I'd bet serious cash your pond has zero ammonia because the fish are alive. But if some "expert" tells you to buy or make more filters the ammonia level will tell whether they're right or full of it. With zero ammonia level you could add 100 filters and the ammonia would still be zero.
Other tests. Endless. You have to choose for yourself how deep you want to go. There are countless ponds out there who's owner have never done any test or adjusted anything. Some of them do get into trouble, just like some people who test get into trouble. Another test option is to take a water sample to a local pet store. I don't like that much because it's easy to contaminate the sample and then you generally have a high school kid doing the test for the first time.
General advice
If you want to make keeping the pond a lot less overwhelming you could slow down a bit. Everyone you ask will have a different opinion and all will be experts. But there is far more misinformation in keeping a pond than good information. Couple that with misunderstandings and you can really get into the soup. This pond you have has probably been running for a long time...there's very little you actually have to do.
For example, (I assume you have goldfish/koi) whoever told you adding salt is important to fish either didn't know what they were talking about or there was a miscommunication. There are people who do believe adding salt is helpful, but it isn't required. There are lots of people who think it isn't a good idea. Higher salt levels cause the fish to produce more mucus which makes it harder for parasites and bacteria to attack. The thinking is more is better. But extra mucus production uses energy the fish might need to fuel other needed functions. Lots of ponds run fine without adding salt.
You're new to ponds, if you make a mistake computing the amount of salt you could kill your fish in short order. Wouldn't it be better to wait awhile, learn the basics first before deciding whether or not you want to add salt? The fish have been fine so far right?
When deciding to do something to your pond I suggest making sure you have at least several points of view. And everything has pros and cons. That way you'll have enough info to make an informed choice that fits you goals, not someone else's. And by several points of view make sure a few of those are outside of online forums. Forums tend to only have one point of view, but you'll hear it a hundred times which might lead you to think it's good advice. Bad advice repeated 100 times doesn't make it right. There are a lot of web sites out there that cite studies, give lots of details and facts you can cross check. Forums are very informal and social. You get more "I did this and it worked, so you should too". That can get you into trouble.
Last home I sold with a pond I explained to the new owners how to keep the pond. They hired an expert. Surprise, surprise it needed everything $$$. The fish were dead within a month of the "fixes". Any chance of keeping in contact with the previous owners? They're your absolute best source because they know your pond.
Good luck. Best thing you can add to your pond is a chair and a glass of wine...for you, not the pond.
