That's just the thing - pressure isn't something that just "happens", it is a feeling, a feeling in the mind.
We can never be certain of when someone else is feeling under pressure - we can guess, based on our own feelings, but we cannot be certain.
And my guess, judging by how batsmen tend to play when they're under pressure, is that it's not often that they feel pressurised by something that's just happened (eg playing-and-missing at the last 2 balls, scoring slowly for the last 7 or 8 overs, which have been suggested by tooextracool and SOC as reasons why batsman always feel under pressure), more that they use the great theory "forget what's happened previously in your innings, it's gone - concentrate on the next ball".