First, find the center of the tree :-)

Looking at the butt-end of the 6x6, note the curvature of the annular
rings. You want to rotate the six-by until the curves are up, the
'cup' is down.

if the log was quarter-sawn the center of the log will be straight
down, relative to the center of curvature. It probably won't be but
the following will still work.

Because a log is a cylinder you can't avoid some curvature in the
annular rings across the width of a spar but you'd like to have them
symmetrical; without any slope from one edge to the other, which is why
you use quarter-sawing. This is because you want the stresses of
flight to be uniformly distributed around the center of the spar. If
their is any slope to the annular rings the center is displaced toward
the down-hill side of the slope. and some portions of the spar's
cross-section end up seeing higher stresses than other portions.

The amount of curvature across a 6x6 will give you some idea of the
diameter of the log. Since you want the flattest possible curvature
you want wood from near the outside of a log about four feet in
diameter. By measuring the center height of an annular ring as well as
its chord (ie, width of the timber) you can work out the diameter at
that particular section of the tree. For 10 rings to the inch , for a
six inch timber, a center height of about a quarter of an inch sez the
tree was about four foot across at that point. Which is good. Closer
you get to the core, the more pin-knots & pitch pockets you'll
encounter. What you'd like to get is wood from the mature growth of
the tree, after it's attained enough height so that it stops throwing
out branches.

So long as the ring count is uniform, anything from 8 per inch on up
will pass inspection but critical builders will often spec a minimum of
10 or 12 rings per inch. Anything more than 16 or so, the weight goes
up faster than the strength, which is why that old-growth Doug fir,
with counts up to 32/in looks better on paper than in use -- the stuff
runs about 40 pounds a cubic foot (!)

If all that checks out, take a look at the run-out of the grain. You
can do this on either surface, once you know how it was sawn. MilSpec
calls for a minimum of 1 in 15, meaning you pick out a grain and follow
it for 15 inches. If it moves more than an inch laterally in that
distance, you look for another stick.

The truth is, minimum grain run-out is determined by the part in the
airplane, in that you'd like to have the grain not run out for the
length of the part. For example, with a 16' spar having a depth of six
inches you'd like to have a run-out of 1-in-32, which would be the
ideal. One in fifteen is a little more than twice that and practice
has shown that's good enough.

In reality, working with wood taken from near the base of the tree (ie,
where the 'cylinder' is fairly uniform), with Sitka spruce it's not
uncommon to see grain having a run-out of one inch in fifteen FEET.
(You really gotta love wood like that :-)

Is it twisted? (If so, you don't want it.) To discover twist, you
need to examine both side of the piece, looking at the position of the
grain relative to the edge and comparing one side against the other.
If the grain is twisted, the run-out on one edge will be different from
the other. That's okay for bridge timbers but when you slice the balk
into spars, they'll tend to turn into pig-tails. Trees exposed to the
wind often have unusual grain patterns -- curves, spirals and so on.
Not the sort of thing you want in an airplane (or a mast).

Compression fractures are easy to spot in freshly sawn lumber (ie, when
you have both sides of the log available for inspection) but difficult
to detect after the lumber has cured. To get it to show up you need to
plane the surface. It's kind of hard to describe what a compression
failure looks like but it gives itself away as an anomolous feature,
usually linear and regular, that cuts across the grain. On close
inspection the annular rings may appear to have been offset by some
small amount and the cellular structure of the wood across the off-set
portion will be compressed. You'll need about a 3x loupe to see this
clearly but the usual inspection doesn't need to go that far since the
other evidence tells you what you need to know.

An honest sawyer won't sell you wood having a compression fracture...
unless you work for Home Depot or whatever. (Then they'll sell you
ANYTHING :-)

Truth is, it's not as hard as it sounds. But it remains a subjective
analysis in which experience plays a crucial role. Fortunately, the
odds are overwhelmingly in your favor.

This has probably been about as clear as mud. Kinda hard to describe
something you learned by actually handling the wood.

-R.S.Hoover

PS -- The liteature takes a stab at explaining 'quarter-sawn' but it's
pretty clear most authors have never stood there and watched it happen
:-)