If your bike is reasonably modern, it will often have an inspection window, low down on one side of the engine. You may have to get close to the ground to see inside properly, and we recommend doing this in daylight, so you can get an idea of the level and the condition of your oil.
The inspection window will have marks at the top and the bottom, and your engine oil level should be somewhere between the two. If the level is above or below these lines, you must act immediately to either drain a little oil away or to top off your oil level.
Just as importantly, you also need to assess the condition of the oil. It should be either green or yellow, as well as shiny and semi-transparent.
If your oil is dark brown or black and you can’t see through it, or you can see sludge deposits, it is definitely time for an oil and filter change.
Alternatively, if you can see metal particles in the oil, this can be an early indication of engine problems ahead; while if the oil looks at all milky, it can often indicate a coolant leak. In either of these two cases, you should consult a mechanic or repair shop to rule out or deal with serious problems as soon as possible.