A hot water boiler gives you a reading you can follow. The pressure gauge is the simplest record of how the system is holding its water, and reading it on a steady schedule tells you more than an occasional check.
Find the parts before you read anything
Locate the pressure gauge, the fill valve, the relief valve, and the expansion tank. The manual labels them, and the boiler page may name the expansion tank type. Know where the valve handles sit so you can reach them without moving furniture.
Read it cold and read it running
Take the gauge reading when the system is cold, then again when the boiler has been running. Write both down beside the season name. The manual marks a band on the gauge, and the needle should sit inside that band. A reading that stays in the band through the seasons is a steady record.
Keep a log you can compare
Use a notebook page or note for the system. Write the cold reading, the running reading, the room temperature you set, and any water you had to add. Keep the entries in order. The log lets you see a trend, and a trend is what you bring to a service visit.
What to do when the needle drops
Water added to a boiler should be a rare event. If the pressure falls between readings, look for the source before you add more. Check the floor and the pipes around the boiler for damp spots. Do not open the relief valve except to test it as the manual describes. If the pressure keeps falling, call a qualified service person and show them your log.
For the typical service life of a boiler and the planning date it gives, see the boiler guide. Use the install year to set that date, and let the log show how the system is aging.