Yes, checking the voltage into and out of the ignition switch right at the switch is probably the best. I suppose it's possible some voltage loss is occurring on the main power feed line from the battery, most likely at the fuse, especially if the fuse holder is the original.
But, if you wanted to clean the switch contacts, luckily they're easy to get at. They're the 1st thing you'll see when you split the switch housing in half. Remove the two screws from the base .....
..... and the upper round black housing that holds the key cylinder will come off, revealing the contacts .....
The white plastic piece with contacts can be pulled off it's post for easier cleaning .....
..... and the detent plate underneath can also be lifted out .....
But you really should consider disassembling the whole thing and servicing it. Most have never been cleaned and lubed since they left the factory, and it's time they were. Here's a recent thread that covers it pretty well. Eventually I plan to write one but I'm still gathering info .....
http://www.xs650.com/threads/ignition-switch-repair.60639/#post-712497