Slow only during overlapping use: what’s the useful clue?

What helps distinguish a restriction near one fixture from a problem farther along a shared drain? If a sink slows only when another water-using fixture is running, does it matter whether that same sink drains normally when used by itself?

I’d also like to know which observations are worth noting without deliberately running more water and risking a backup. For example, if other fixtures seem normal and no nearby drain has water rise in it, does that meaningfully narrow the possibilities, or is professional inspection still the sensible next step?