Bearing up

One or both of my rear wheels have been making rumblings.
I’ve been alternating between thoughts of worn tyre noise and bearing noise.
Jacked up both wheels today and the bearings felt fine - but I know it’s hard to examine bearing smoothness unless you take the wheels off and I didn’t have time for that. Tyres look fine though.
Anyway if whatever is awry lasts for another 1500 miles then I’m not too bothered.
I’d just like a guarantee.