Lay a straightedge across the top, then very carefully look across the front, changing your viewing angle so you are looking at both the straightedge and your bars. When the sliver of light between them is the same, so are your levers.
The other is using a bubble level. Prop your bike until the bars are level, then put the level on the top of the levers.
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Last edited by RoadTire; 06-01-14 at 03:41 PM.
Reason: really crappy first daft .... umm, draft.