Gradual wear, sudden consequence
Sinkers and cams work against the needle on every course. They wear at a rate set by yarn type, speed, running hours and lubrication — and because the wear is even across a machine, there is no obvious moment where one part is clearly worse than its neighbours. That is what makes them easy to leave too long.
What to look at
The working surfaces and edges, for rounding, scoring or pitting. The fabric, for a change in appearance or weight that does not trace back to the yarn or the settings. Needle consumption — a rising needle-breakage rate with no other explanation often points at the parts working against them rather than at the needles themselves.
Replace by measurement, not by feel
The manufacturer's wear limits are the reference; running hours are the planning tool. The useful discipline is to measure at scheduled intervals and record the result, so replacement is a decision made from a trend rather than from the day somebody happened to look. If you do not have the wear limits for your machine, ask us — that is a question with a definite answer.