Airspeed Indicator Accuracy Tolerance

In my Cessna 421C, the pilot-side airspeed indicator reads 6 nts below
the co-pilot-side airspeed indicator. When I pointed this out to a
mechanic, he said that it was "within toleraance." He said it was
possible that one was 3 nts high and the other 3 nts low and so both
could be within tolerance. He added that there is no real internal
adjustment, so there is not much he could do about it anyway.

Seems to me that when you are talking about a possible 6nt difference
and given the need for accurately maintain blue-line speed in case of a
single-engine failure, it is important to know that one's airpseed
indicator is accurate. Isn't there a way to judge true accuracy of
speed indicators (just as there is for altimeters)? Should I be
satisfied with my mechanic's answer and move on?

-Sami
N5554G, Cessna 421C