Load it up and measure the weight on each wheel with some bathroom scales?
When my Streetmachine's fully loaded the balance point (for lifting, not that I can really do it) is somewhere between the wheels, around the base of the seat. I'm not entirely sure what adding the rider does to that, but I reckon there's a noticable front-wheel bias with rider and no luggage. YMMV on this one, particularly where body-shape interacts with seat and boom position.
I tend to inflate both tyres to the same pressure (chosen primarily on the basis of the comedy off-roading factor). They're Marathons and not particularly fussy, and the suspension does much of the work.
(The Red Baron is a rigid racing machine that rarely carries more than a few kilos of rack bag + water
[1] and is *extremely* sensitive to tyre pressure on the front: There's a fine balance between mushy steering and shaking your ankles off on bad roads. I'm not sure what the actual front/back weight distribution is with the rider on board, I try to avoid limit-of-traction situations on that one
The ICE trike has a very lightly loaded rear wheel - without luggage traction can be an issue. Most of the weight is split between the front wheels, so pressures are lower than a SWB bike.)
[1] I loaded it up with camping kit for racing at the York Rally (the train took most of the strain), and discovered the handling was *scary*.