I guess it goes without saying I'm not an engineer or contractor. A
friend who's an unbelievable DIYer asked me if a (small) bathroom wall
I plan to tear down is load bearing. I couldn't answer. He asked if
my home is stick-built and if the attic has trusses.

There's really only one way to find out. Climb up into the attic and see
what's resting on top of the wall, then climb down to the
crawlspace/basement and see what that wall is resting on.

If the wall is running parallel to the ceiling joists or floor trusses,
it's not load bearing (unless you see something in the attic resting on top
of the wall). If the wall is running perpendicular to the joists/trusses,
you'll need to investigate further.

If the wall is truly a bearing wall, the wall will be transferring that
load to something UNDER the wall. If you only have floor joists under the
wall, it's unlikely it is is load bearing. If you have a beam or support
wall in the crawlspace running the same direction under the wall (or offset
a foot or two either way), there's a good chance that wall is supporting
some kind of load.

If there's a door in this wall, another indicator would be to check the
framing in the wall above the door. Bearing walls will need a header above
the door, where partition walls are typically framed without a header. You
could check this with a studfinder to see what framing is inside the wall
above the door. Having a header doesn't necessarily mean the wall is load
bearing (I put headers over all my doorways, even when they weren't
needed), but if the header isn't there, it's unlikely it is bearing any
weight from above.

If you have any doubts, hire a pro to come have a look!

Anthony