...You're supposed to have the bike upright when checking the oil so while filling and checking,...
on all the bikes that have the quarter sized sight glass window to check the oil, get a little piece of mirror, sit on the bike on level ground, keep the bike in gear, and with one hand on the handlebar (grab the brake if the glass is on the left) push the bike up straight and reach down with the mirror to see the window.
You can lean the bike a bit left and right and watch the oil go up and down in the window. Where exactly the bike is vertical is subjective. This will give you a better feel for where the oil level is.
The plate behind the window is white. When the oil is dirty its very easy to see the black oil against the white plate. New oil can be a little harder to see, esp if you run synthetic oil, it tends to be yellow/gold and does not stand out as well.
The other thing, when you change your oil look in your owners manual to see how much you need, depending on whether you changed the filter or not. On all the Vstar bikes I have looked at you only need to change the filter on every other oil change: 4000 miles for oil, 8000 miles for filter. Whatever the owners manual says, put that much oil in the bike.
If you try to watch the sight glass while you are filling it up after an oil change, it will look full about 8 ounces short of what it needs. When you start the bike up it will pump that oil into the transmission and up in the heads, and it will read low. If you watch the glass again it will take another 5 ounces to make it 'look' full, and when you start it up again, it will be 3 ounces low.
Just put the amount in the manual says, check the sight glass to make surer you did not miss a quart, it should look like its overfilled. Ride the bike and check it when you come back, after it sits for a minute, and it should be fine. If its low after that, you are leaking oil.
If you dont have the owners manual for your bike, they are free online downloads from yamaha/vstar - easy to find.
Also the factory service manuals for most vstar bikes have also been scanned into pdf format, and are easy to find online as well. Some of the pages are a bit blurry (like the schematics/wiring diagrams) and the index table of contents links dont work in the scanned copies, but its better than nothing.