Often, first symptom was difficulty priming, so was making that rattly sound that you hear when there's an air bubble. With mine, I'd cleaned it, put everything back together, got it primed. I cleaned up and went to bed. Got up in the morning and it was making that funny noise, like there was an air bubble. There was no way to get it priming properly. Bad enough for some air to get in, but not bad enough for a water leak.Should I see air bubbles? Or noise?
Or cracks in the rubber?
Or will it become less difficult to remove the head?
Will the canister just get a reduction in flow?
If it gets bad enough that you can see cracks in the rubber, or see it bent out of shape, chances are, you would notice at least some water leak.
If you suspect a faulty O-ring, you can rub some silicone gel on it, generously. If it resolves the problem; great...it will work for now, but it's only a band aid fix.