A motorcycle chain and its sprockets wear together. That is why fresh lubricant or correctly adjusted chain slack cannot, by themselves, establish that the final drive is healthy. Teeth on a sound rear sprocket have a fairly even shape; as the system wears, they can develop a pronounced hook. That change is a clue that the chain rollers no longer sit against the teeth as intended.
The way the chain sits on the rear sprocket provides another visible clue. If it can be pulled away at the back far enough to expose a clear gap beneath a link, the drivetrain is already heavily worn. Look also for links that remain stiff and for the O-ring seals between the chain plates. Cracked or absent seals cannot keep lubricant in and dirt out as they should.
Inspect only a stationary motorcycle with its engine off, and use the owner’s manual for the limits that apply to the particular model. If the teeth, links or seals show marked wear, adding more lubricant is not a repair. When a worn final drive needs replacement, chain and sprockets are generally changed as a set: old sprockets can shorten the life of a new chain.