If you believe the
levels of consciousness scale Steve posted on his blog has anything to do with an individuals vibration, I'd say you could use the particular actions an individual takes and their behaviour as an indication of their vibration. Eg. Actions associated with levels such as
anger or
fear would generally indicate a fairly low vibration, while those associated with
acceptance would indicate a fairly high vibration. Of course, you have to be careful not to mistake a conscious display of low-vibration related actions with a low vibration (as people at higher vibrations can sometimes display the actions related to a lower level of consciousness/vibration at will without sinking to the levels associated with those actions... usually used when interacting with others to help them in some way, prove a point, or generally get through to them).
While I'm aware that it is possible to temporarily raise your vibration, from experience, such a thing is not sustainable for long periods of time unless you have the necessary "foundations" supporting that level, so again, if you subscribe to the levels of consciousness/vibration belief, I’d say a snapshot of someone’s actions would give you a pretty good idea of where that person is at if you consider the range of actions/behaviour you’ve seen and average it out.
I’m sure there are better/more efficient ways of doing it, but I thought I’d put this forward as a more down to earth thing that non-psychics and intuitive individuals can use to discern vibration to a reasonable extent.