when do you know you need to replace your leaf springs and torsion bars?

when your butt is draggin too low or your fronts too soft for your liking

Tb, when you are needing stiffer TB's, or when you likely have to often readjust TB ride height screws often. On leafs, when you have to re arch springs more often then you prefer. They don't really wear out in any kind of normal use. Others will have their own likely different opinions.

I think the rear leaf springs were supposed to sit nearly flat? If the center of the leaf spring is higher than the ends, then I am pretty sure the spring is shot.
Torsen bars are tougher to visually inspect because ride height can be changed with the adjusting bolts. So you would have to go more by ride quality. If the car bottons out easely going over dips in the road, or leans really bad going around corners then you will want to get new torsen bars.
On my Charger, I went with the larger torsen bars and had a shop re-arch the rear springs while adding an extra leaf. The suspension is pretty firm, but not really much stiffer than some newer cars I have driven.