Boiler pressure does not usually fall in one dramatic step. It drifts. The early signs are small, and they show up most clearly when someone reads the gauge the same way each time and writes the result down. This list is a set of things to notice and record, not a diagnosis.
Read the gauge the same way each time
Check the gauge when the system is cold, at the same point in the season, and at the same time you check anything else. Note the reading in your log with a short line about what you saw. A reading taken on a warm system can look different, so keep the conditions consistent.
Early signs to log
Log each of these when you see it. Needing to add water to the system more often than before is the first. Note how often you add water and how much the gauge moves when you do. Watch for a damp spot near a fitting, valve, or the pipe connections, even a faint one. Note radiators that heat at the bottom but stay cool at the top, which can mean air or sludge in the circuit. Record any discharge from the pressure relief valve, and any new knocking or gurgling sound when the system starts.
Keep the trend, not only the reading
A single low reading may mean little. A line of readings that drifts down across a season tells you more. Keep the log with the boiler's service records so that the next visit starts from your notes. Add a line each time you top up or find something new.
When to call the technician
If the gauge keeps falling, if water is needed often, or if any of the other signs appear, take the log to a qualified heating technician. They decide on the repair. Your part is the record, and a clear log lets them start the visit with the facts already in hand.
For the typical life of a boiler and how to plan around its age, see the boiler page.