Runs are, by definition, maximal: a clue of 3 means exactly three filled cells with non-filled cells (or a wall) on both sides. So the instant you can prove a block of filled cells corresponds to a completed clue, the cells immediately before and after it are empty — no further analysis required. Placing those marks is called punctuating, and skipping it is the most common way solvers leak progress.
The subtlety is knowing which clue a block belongs to. A block of three in a line whose clues are 3 5 might be the finished 3 — or the middle of the growing 5. Only punctuate when the block's identity is pinned down: it is anchored against a wall or an X through the chain of earlier clues, or no other clue of that length could occupy its position. When a block touches a wall, identity is automatic: the first block against the left wall is the first clue, full stop. (The in-game clue strikethrough on this site follows the same anchoring rule — a clue only greys out when its run is provably locked, not merely when some block matches its length.)
Punctuating is also the engine that moves information between lines. Every mark you place lands in a column (or row) that now has one fewer unknown, and crossing-line bookkeeping is where nonograms are actually won: fill or mark a cell, then immediately glance at the line it crosses. A fresh X can split that line (see splitting); a fresh filled cell can glue a run there (see glue).
When an entire line completes — every clue accounted for — punctuate the whole thing: every remaining cell is empty. A fully sealed line stops being a puzzle and becomes a wall of fixed facts for all the lines that cross it. Sweeping a just-finished row into its twenty columns routinely unlocks a cascade of new deductions, which is why experienced solvers finish lines greedily rather than leaving them at 90%.