Why There's No Reliable Signal
Fibre intake doesn't produce a specific, identifiable feeling the way, say, obvious dehydration might. Whatever someone might attribute to "low fibre" could plausibly come from many other dietary and lifestyle factors, which makes self-diagnosis from how you feel an unreliable method. Two people eating the same low amount of fibre can feel completely different day to day for reasons that have nothing to do with fibre at all.
The Honest Answer: You Count It
The reliable way to know is arithmetic, not intuition: add up the "Dietary Fibre" figure from the nutrition panel on packaged foods eaten over two or three ordinary days, plus a reasonable estimate for fresh produce, and compare the total to NHMRC's Adequate Intake (30g for men, 25g for women). It doesn't need to be exact to be useful. Most people who do this once are at least roughly accurate about where they land from then on.
Why This Matters
Given most Australian adults don't meet the benchmark, the higher-probability assumption for most people is that intake is lower than the target, not that it's fine. Counting it for a couple of days is a more useful starting point than waiting for a sign that may never clearly arrive.