OK... so this is a little embarassing...

Just about anytime I take a crap in one of our toilets and the load is
"large" the toilet tends to clog. It happens almost all the time if I
use toilet paper but still clogs often even if I flush before
wiping. [sorry for the details]

I can always clear it with a few quick plunges. I have tried using one
of those hand screw augurs (the ones with the plastic sleeve to
protect the trap) and it didn't meet any resistance nor did it fix the
problem. I have also tried feeling down the trap with my hand and
didn't feel any objects there.

The toilet itself is a regular gravit flush one, maybe 5-10 years
old. I can't see the model number.

Where is the likely source of the problem? clogged/bad trap? too
narrow waste pipe? too many bends in the waste pipe? bad wax seal? bad
vent?

I can't see the waste pipe so I don't know the diameter but since this
is a 150 year old house, I wouldn't be surprised if there were issues
somewhere with the plumbing. By the way the bathroom (and toilet) is
on the third floor and I suspect that it was plumbed in sometime in
the last 30 years

Any thoughts on how to troubleshoot this? I would prefer not to remove
the toilet if I don't have too...