This is exactly my concern. Sometimes you only notice the difference when you place new and old filters side by side and the difference can have considerable impact on the print.
Equally, a new enlarger bulb. Not only the light intensity but also the light colour changes significantly and degrades exponentially with use and age. So just because it turns on doesn't mean your getting the necessary quality of illumination.
And it goes without saying, using decent multi-coated enlarger lenses.
It's easy to take all these seemingly little things for granted but it can turn really frustrating darkroom sessions into very satisfying and productive ones, just with a little bit of investment.