Look Closer. All Harleys made after '07. I know, have one. It is either on the nacelle, in the fairing, or that thing on the left that looks like a fuel cap, but big label: "Do Not twist--Not a fuel cap". They also have a below 1/4 tank orange warning lite on speedo. Some Sportys had a LED bargraph gauge you simply did not Recognize. All Feulies do, too, as they don't have a fuel tap with reserve.
My '83 Honda has a digital bar graph gauge. If you don't have one, fill up, reset trip meter. ride until you have to turn on Reserve. Then, note the miles. If it is ,say 140, then get gas at 110 or so. Or carry a quart of gas in an old oil bottle.
On some you can just look down or use a stick. Some, any splashing means a pint or more left. Some, you really can't see if any gas in. My Honda, even if you hear splashing, that could be fuel trapped on right side, which is prevented by the big frame well from getting to the L side where tap is.