Its possible to detect (***not measure***) as little as .003 of an inch by touch with large shaft and bearings
Oil will mask this so its very hard to detect on a plane bearing. Odds are if you can feel movement you have problem......
When I was a winder with golden hands ( younger and stronger back too ) I could feel the .003 clearence on a C3 bearing and make a judegernt call on how tight things were.
What didn't feel right durring fitting would have to be followed up with a mic and odds were if you could feel you had a problem.
Now I don't do this for a living anymore and I don't have the golden hands I once did.
However if you start from a good bearing and as part of your Isnp/PM routine touch, poke, prod and feel as meany moving parts as possible you can learn what things should feel like. A Stethiscope is also a fantastic tool for doing you Inspections, if your a little hard of hearing ( like me ) you can focus right in on parts and places and listedn for thuds or unusual noises.
In almost everything ( rotatign electrical machine, gear box ect ) I have ever done PMs and inspections on I have found they don't just fail, they anounce themselves long before failure happens.