looking for advice to mitigate inner groove distortion

I hear it mostly in horns and vocals.  It's a fuzzy halo around the sound of the instrument.  Also in loud passages of orchestral works that occur in the inner grooves.  I start to hear it about 3/4 of the way from the edge of the record to the end of the grooves and it gets worse as it gets closer to the center.  It's worse on some records than others and sometimes, I think it's the record that's the culprit but it seems that the turntable and cartridge are playing a role as well.  I checked for anything obstructing the travel of the tonearm but I didn't find anything.  I tried adjusting the anti-skate.  Some fine-tuning helped a little (maybe, not sure I trust my ears sometimes) but it was still there 

What tools are you using to ensure that your cartridge alignment and calibration are correct?

Both channels?  Or one channel more than the other?
cleeds, I think your thought about alignment is reasonable, but as was noted elsewhere, misalignment is not usually that obvious in my own experience and that of others.

Also, where is the motor located on the Rega P3?  Is it under the platter or outboard and perpendicular to a line drawn from the pivot to spindle?

cleeds, I used an alignment protractor and my eyeballs :)  I used a tracking force gauge to calibrate my tracking force.  The anti-skate on the P3 is pretty primitive so that part is imprecise,

lewm, both channels and the motor is located under the platter in the center and on the rear side of the plinth

adam8179 

I used an alignment protractor and my eyeballs ...

Not all cartridge alignment contractors are equally precise.  Many align the cartridge itself, which isn't ideal. The best alignment gauges, imo, align the cantilever. They're much trickier to use, but I think worth the effort.