Most water treatment equipment has gauges, indicator windows, or simple flow behavior that changes gradually as the system works. You do not need tools to notice the change. You need a habit of looking at the same few things on a regular schedule, and a written baseline to compare against.
Start with the baseline
Ask the installer, before they leave, to show you the gauge readings they recorded at startup and to write them down, or to confirm in writing what the normal operating range is for your unit. If the unit has no gauge, ask the installer to note the pressure at the nearest faucet with the system running and nothing else drawing water. A reading without a comparison point tells you very little later.
What to look at each time you pass the equipment
Check the pressure gauge on the inlet and outlet sides if your system has them. On a filter with two gauges, a growing gap between the two readings usually means the filter media is catching more material than before. Note the date, both numbers, and whether the gap is wider than your baseline. Check any indicator window or flow meter the manufacturer included, and note its position. Look at the housing and fittings for damp spots or mineral crust, and look at the floor beneath for drips.
Check the flow at the tap
Pick one tap that is farthest from the equipment and run it at full flow for a set time, such as the time it takes to fill a pitcher you already own. Do this at about the same time of day each visit, because household demand changes. If the fill time is getting longer over several checks, something upstream of the tap is restricting flow, and the restriction is worth reporting.
What not to do
Do not open a pressurized housing to see inside. Before you touch any filter housing, shut off the water supply to the equipment and relieve the pressure using the method in the owner's manual. If the manual does not describe the procedure, ask the company to walk you through it at the next visit.
What to bring to the service call
Your notes are the most useful thing you can hand a technician. Bring the readings, the dates, the baseline from the installer, and a plain description of any change, such as a longer fill time or a new drip. Ask the technician to tell you which reading would mean the element needs replacing and which would mean the problem is somewhere else in the plumbing. That answer turns your checks into a decision you can act on.