Look down into the feeder reservoir through the open filler cap.  You can normally see the syrup level with a simple sight-check like this.  If you especially have a lot of shade, or if you are refilling the feeder at night, you may either need a flashlight or you may use some sort of dip-stick to check the syrup level.

Refill the Feeder

If you refill when the feeder still contains syrup, be especially careful to not overfill the feeder, as excess syrup will run over the lip of the feeder reservoir and into the hive body.  This may cause robbing, attract ants into the hive, and could disrupt egg-laying and brood production for a day or so.  However, the Dual-Access Hive-top Feeder will hold over a gallon of sugar syrup.  The easiest way to ensure you never overfill the feeder is to refill it only when it is empty, and always use a gallon of syrup.

After you refill the feeder, replace the filler cap using only fingertip-tightness.

Tip: If the wind is blowing when you are refilling, you may want to shield the flow of liquid from the wind with your body to keep the wind from blowing the liquid onto the top of the hive.  You may also use a funnel to minimize any spilling.

Never grip the cap tightly or use a tool to screw the cap down hard onto the filler tube.  Only fingertip-tightness is required!  Full weather and insect sealing is accomplished by tightening the cap with the tips of your fingers until it is just snug on the filler tube.

A good rule of thumb for tightening the cap is to start the threads and tighten until you begin to feel it become resistant to turning with your fingertips (approximately one full turn).  At that point, stop tightening.  

Forcing the cap down more than fingertip-tight makes it unnecessarily difficult to remove later, could break the weatherproof seal around the tube, and does not provide any better protection for your hive.