Simply, you have to notice the change without looking out for it.
That is probably the best way to ensure you do not bias or trick your perception, or even induce a placebo effect (admittedly beneficial in itself if you can sustain it).
Whether eating a tonne of beetroot for its anti-oxidants and allegedly improve brain function, or drinking more water to feel less sluggish, don’t pay yourself any attention looking for signs of improvement. The human body is an exquisite barometer of change, and it lets us know of even the tiniest shift towards either ease or disease. When the balance is upset, we cannot fail to notice it – sometimes catastrophically, more usually gently – as there is nothing more intimate or unceasing than our bodily sensations and mental processes.