For daily or ongoing measurement, the mirror.
I get calipered only a few times per year
as reality check or re-calibration of my
eyeball estimates.
This is very workable when in the 8% or lower
bf range. For someone who might be 12, 15, or
20% – they don’t know – the thing to do is
to get a caliper measurement. It’s possible
that to be much fatter than you think. (Three
or four years ago, I was actually 19%… had
no idea I was THAT fat.)