A faucet tip is one of the easiest places in a house to read the water. The aerator is a small screen of mesh and plastic at the end of the spout, and whatever the water leaves behind collects there first. The crust you find on it does not tell you everything, but it narrows the list fast, and it costs nothing to look.
Look at the crust before you blame the softener
Unscrew the aerator by hand or with a cloth-wrapped pair of pliers, set the parts in a dish, and look at the mesh and the inside rim. Compare what you see with these common patterns.
White, chalky, or flaky deposits that scrape off with a fingernail usually point to calcium and magnesium scale, the mineral load that makes water hard. This is the pattern most people mean when they say their water is hard.
Orange or rust-brown staining that comes off unevenly, or that also shows in the toilet bowl and on sinks, is more often iron. Iron is its own problem, and a softener does not solve it on its own. Get the water report before you decide anything.
Black or grey grit, or fine sand that rinses out of the screen, is sediment. It can come from the supply line after work on the street or from a water heater that has not been flushed. It is not scale, and vinegar will not dissolve it.
Green or blue-green staining, especially on the hot side, can indicate copper corrosion. Treat that as a plumbing question for a licensed plumber, not something to solve with a soak.
Test hot and cold separately
Check the cold tap and the hot tap on the same sink. Scale that builds up mainly on the hot side points toward the water heater, where heating pushes minerals out of solution and onto the element or the tank floor. Scale on both sides points to the supply, which is the condition a softener is designed to address. If only one side is affected, be careful about concluding the whole house has a hardness problem.
Soak it, then watch the next few weeks
Soak the aerator parts in plain white vinegar for an hour or so, rinse them well, and reinstall. Clean parts tell you little on their own. What matters is what comes back. Mark the day you cleaned it, wipe the rim, and look again in a week or two. If a fresh white crust returns quickly on the cold side, you are seeing a steady mineral supply, and that is the pattern to bring into a softener discussion. If the aerator stays clean, the earlier build-up was probably a one-time event, such as sediment after work on the line.
What to ask before you call anyone
If you have a clear pattern, bring it into the conversation. Ask a company what the crust would tell them, whether they want a water test before they quote, and whether their recommendation would change if the deposit turns out to be iron, sediment, or copper. A company that gives the same softener answer no matter what the deposit looks like is telling you something. Keep photos of the aerator parts with your notes so the next technician starts ahead of you. For how this site weighs companies on these kinds of questions, see How We Rate Water Softener Companies.
When to stop guessing
If the crust is orange, green, or heavy on the hot side only, or if the water has a metallic taste, stop cleaning and get a plumber or a water test. Hard water scale is common and manageable. The other deposits need a different answer, and the sooner you know which one you have, the less you will pay to find out.