Usually sugar in syrup is measured using a tool called a hydrometer. It looks like a very fat thermometer. It'll float in the syrup and how deep it floats will show you on a marker what the density of the syrup is. Then you look up that density on a syrup making chart (onlinelibrary.wiley.com/doi/10.1002/9781444314748.app2/pdf) to see what percent sugar it matches.
To test the sugar content in maple sap, you need to use a Sap Hydrometer which can either be tall or short. Then, you can hold the hydrometer firmly until it's floating so that you can be able to read the number on the surface of the test. For reference, go to http://andersonsmaplesyrup.com/index.php?page=sap_hydrometer.