The dip stick will work well... if the bottom of your tank is flat and level. However, most motorcycle gas tanks are much lower in the back than in the front where the filler tube is. If you probe with a dip stick or string and washer in the filler tube at the front of the tank you will not get a correct reading of what is in the tank. In fact, you could touch the bottom of the tank and find no gas at all... but there could still be more than 2 gallons at the lower back end. On most gas tanks with a reserve, the tank is designed to still have about a gallon of gas when you have to switch to the reserve. This gives you 40-60 miles depending on your MPG and how much reserve the tank has. The best way is to look up how much gas your tank holds. Fill it full and check your odometer. Go 100 miles then fill full again and calculate your MPG. Then multiply your MPG by how many gallons your tank holds. Leave a safety margin, and you will always know how far you can go. Whenever you get fuel, always fill it and reset the trip meter, and check your petcock to make sure it is in the on position and not reserve. You will always know how far you can go, and have the reserve for emergencies.
On a side note: not all gas gives exactly the same MPG, bio fuel, methanol, and other additives will effect mileage. In addition, riding habits (racing, high rpm, or babying it) and conditions will also greatly effect MPG.
If your really worried about running out of gas, they make portable fuel containers and fuel cells for motorcycles. Pick one up and carry extra gas if it gives you peace of mind. Worrying about something all the time on the bike takes the fun out of riding it.
Ride safe, ride fun