I suspect most weaknesses are addressed at least in part by climbing on a board with terrible footholds (fingers, core, power to varing degrees, depending on the board and problem setting). The weaknesses that aren't, seem to require far less fun activities such as 4x4s and laps on easy routes - basically tedious mileage and learning how to rest.
In terms of identifiying specific weaknesses, asking people who see you climb is probably the simplest means. I once did a "blind" Lattice assessment, essentially as part of a UKB "bet" about the power of the Lattice method. Tom Randall tested me, and also asked people who knew me for their opinions on my strengths (ha!) and weaknesses. He then did an assessment without Lattice's usual pre-knowledge of actual levels (although, I heard afterwards there were a few cheeky, informal enquiries).
His views based on just seeing me climb were more accurate than the ones after lots of number crunching, and the opinions of friends were probably even more to-the-point (verging on contempt!). Mind you, I was / am (wrist injury notwithstanding) a bit of a freak - and perhaps my defects are more visible (and yet less readily resolved by maths!).
