Most people find out how a product suits them by putting it on their face and waiting to see what happens. It works often enough that nobody questions it. The trouble is that when something does happen, the method gives you almost nothing to work with. Here is a slower approach that produces an answer you can actually use.
Start small and away from the face. A small amount on a small area of the inner forearm, or behind the ear, is enough. Both are easy to keep track of and easy to leave alone. Use roughly the quantity you would normally use, not a thick layer, because a thick layer is not the way you would use the product and will not tell you about the way you would.
Then leave it for twenty four hours and do not wash the area. This is the part most people skip, and it is the part that carries the information. Washing resets the test. Reapplying resets the test. The point of the twenty four hours is that the product stays in contact with one patch of skin for a full day, in the ordinary way, without anything else happening to it. If the area becomes uncomfortable at any point, wash it off. The test is over and you have your answer.
If nothing happens, repeat the same thing on a small area of the face for another twenty four hours. Facial skin behaves differently from the forearm, so the second step is not a formality. Pick one spot, apply the same small amount, leave it alone for a day, and look again.
Introduce only one new product at a time. Two at once tells you nothing about which one caused what, and it is remarkably tempting to start two together when both arrive on the same day. If you do, and something changes, you are back to guessing, and the only way out is to stop both and start over. One product, a clear window of time, then the next.
It is also worth separating two different kinds of observation. Something that shows up on first use and something that shows up after six weeks are not the same event and rarely have the same explanation. The first is an immediate response to a product that has just arrived on your skin. The second happened while dozens of other things were also going on: the weather, the water, sleep, other products, the season. Six weeks in, the product is one of many variables, and treating a late change as if it were an immediate one leads people to blame the wrong bottle.
Keeping a note helps more than it sounds like it would. The date you started, what you used, and what you saw. Memory reorders events to fit whatever explanation arrived last, and two lines written down at the time beat a confident recollection a month later.
None of this is advice about anyone's skin, and none of it says what anyone should or should not use. It is a way of getting a clean answer out of a test instead of a muddled one. This applies to anything you buy, including ours.