Small amount of rusting is treated as good for bonding with concrete by many.
When its start to scale it is not good, in scaling the wearing of steel can be seen.
to prove you can do following test by removing rust from it;
Checking the diameter of steel within tolerance.
Checking weight per meter within tolerance.
Doing a physical testing for strength property of the steel.
Rusting is a process of oxidisation of iron (steel) and it will start decaying it from the surface.
The small amount of rust like brownish colour actually reduces the rate of further rusting due to layer created on surface.