The honest answer: in real time, you cannot. A correction counts as finished only in retrospect, once it is visible that what followed has a different structure. At the moment you are looking at the chart, the last leg may be unfinished, or it may be the start of the next form.
That is not an evasion. It follows from the definition: a form is identified by its internal build, and the build of an unfinished leg is not yet known.
What can be stated instead
Three things, all checkable.
The level that invalidates the count. Every version has a price beyond which it becomes inadmissible: a wave cannot go beyond its own start. That is the most useful thing markup provides — not "the correction ended" but "if price reaches here, this version is wrong".
The number of legs. Three completed counter-trend legs are the minimum for a simple correction. While there are two, talk of completion is premature.
The structure of what follows. If the move from the presumed end divides into five and runs with the trend, that argues the correction is behind you. If it divides into three, it is more likely the correction continuing as a combination.
Why a Fibonacci zone is not a criterion
It is tempting to call a correction finished when it reaches a typical proportion. That fails for two reasons.
Price regularly falls short of a zone and regularly overshoots it — proportions describe the typical, not the obligatory. And one situation usually has several admissible zones: one for a zigzag, one for a flat, one for the next degree up.
Reaching a zone is an argument, not an event. The event is a break of structure at the appropriate degree.
Combinations: why the ending gets postponed
The most common reason "the correction has ended" turns out wrong: one ABC is followed by a connecting leg and another ABC. Formally nothing is broken — the correction simply proved more complex than it looked.
There is nothing to foresee that with: combinations have no geometry constraining their length. So "the correction may continue" is a description of the situation rather than a figure of speech.
How to use this
Hold two versions — that the correction is over and that it is not — and know which price cancels each. That is less comfortable than certainty, and it matches what is actually known.
A common mistake
Calling the end of a correction on the first impulsive move up. The first leg of a new wave and wave B inside a continuing correction look identical — and differ only in what happens next.