With the bike not running and upright on the centre stand or a paddock stand you should be able to see the oil through the window. If the window is black then the oil is what you are seeing and it probably needs changing. If you see nothing except the inside of the sump (silver coloured metal) then you need to add some oil.
I'm not a mechanic and this is my first bike but I'm guessing that if you had a leak and most of the oil leaked out without you noticing and you were riding it for some time, you may have damaged some of the internal seals. This may be why you are blowing so much smoke.
If there is too much oil in the bike this may also be causing the smoke. From completely empty with a filter change you need 2.4 litres of oil according to the manual, mine took closer to 2.6 but that just may mean that the markings on the oil bottle aren't all that accurate.
This is an image I found on google of a different bike but you get the idea as the inazuma looks about the same when looking at the oil through the window. It should be easy to see the oil level.
Best of luck.