My bike has a sight window for oil and no dip stick attached to the dip stick like nob for filling. I have had the bike from new and the manual ( handbook does not mention oil checking).
I am at a loss to know how to check this, should there be a dip stick? Should the oil be up to the tip of the window?
Help please 
Yes the ‘sight glass’ is a complete pain. First of all, bike must be upright, so for lack of mainstand I used to sit on it then reach down with a small mirror to see into the sight glass. Correct level is in the middle of the round window. (Also the oil is quite awkward to even see if it’s new.)
Since then it has a sidecar attached, which keeps the bike upright, which is great, but then I can’t get a dead-on line of sight of the window because the sidecar is in the way. So what I do now is lean over the seat and tank from the right side, with the small mirror and a torch. I kid you not.
I did convert a plastic Renault dipstick to act as dipstick, but to be honest it’s a bit vague so only sort of works, so that the sight glass is more reliable.
HTH
I crouch by the sight glass and set the side stand on the toe of my boot while holding the bike, i can rock it slightly away from the boot which is then level, I can hold the bike with one hand while adding oil from an oil can with a thumb lever to pump it in via the long tube/nozzle on the can.
when I first got the bike i leaned it against a plastic coal bunker and poured oil in from a measuring jug and a funnel but the oil can is easier.
oil should be checked when warm, with the wheels on the ground, not on a centre stand.
there are two marks by the side of the window for low and high. I fill to approx. 2/3rds usually.
For me this is the key advice on any model .As soon as possible, when the bike is in your possesion, dump the oil that’s in there, add the correct oils in the correct amounts as per the owner’s manual, and then make your own marks on dipstick, sight-glass, etc. and check levels in a consistent manner (on center stand, leaning against the coal bunkerl, or whatever works for you).
My 2008 Nevada has a dipstick, great! But handbook requires …clean dipstick, screw fully in then screw out and check. Problem is that by the time you have screwed out the warm oil has run down the stick to give a false low level. So I just dip the level without screwing in and adjust by the length of the thread
Yes so do I, not difficulr to do. I also concur with refilling with a known amount from a measuring jug. 