...or you can also measure it from a compass rose at a VOR. On en-route charts the isogonal looks different to the sectional, it is a thin green line on the en-route chart. Or you can get it from the A/FD (now called chart supplement) where the variation will be in the 'navaids' data for each airport.
I remember it was sometimes hard to find the variation information quickly on the test because you only have a small part of the chart and sometimes they are not very good quality.