Most people notice a filter problem when the floor is already wet. The earlier sign is smaller: a film of moisture, a faint ring of mineral dust, or a single drop that forms on the bottom of the filter bowl after the water has been running. That is usually the O-ring, the rubber seal between the bowl and the head, and it deserves attention before it becomes a leak.
What the early signs look like
Dry the housing completely with a towel, then check it again after the system has run for a while. A seal that is starting to fail often shows a slight wet line on one side of the bowl rather than a steady stream. Look for grit or white residue around the rim, which can mean water has been seeping out and drying. A ring that is flattened, cracked, or has lost its sheen is worn even if it is not leaking yet.
Check the seal and the groove
Shut off the water supply and relieve the pressure first, following the maker's steps for your housing. Remove the bowl and look at the O-ring. Feel for flat spots, nicks, or stiffness. Then inspect the groove where the ring sits. Sediment or hardened scale in the groove can push the seal out of position even when the ring itself looks fine. Clean the groove gently, without scratching its surface.
Replace it the right way
Use the replacement seal specified for your housing. A generic ring of the wrong size or material can swell, crack, or fail sooner. Put a thin film of the lubricant the maker recommends on the new ring, seat it fully without twisting, and tighten the bowl by hand until it is snug. Over-tightening can crack the bowl and cause the very leak you were trying to prevent.
When a drip means something else
If the wet spot returns after a new seal, the cause may be a cracked bowl, a loose head fitting, or a cartridge seated at an angle. A slow drip that appears only when the system is under pressure can also come from an upstream connection. Stop using the system until you can see where the water starts, because a hidden leak can soak a cabinet floor before anyone notices it.
What to write down
Note when you replaced the seal and what you saw at the time. A drip that returns at the same spot within a short period points to the housing, the fittings, or the water conditions needing a closer look. A short written record also makes the conversation with a service person faster and clearer.