Generally, it's not a problem until you start adding longer shackles up front. You still have the stock shackles, so that wouldn't be my first guess. Of course, this all depends on how well those lift springs were built. Sometimes the flat area surrounding the spring center pin just doesn't sit the axle at the right angle and causes caster problems.
Caster is pretty easy to measure with an angle finder.
Also, to answer your original question, measure the distance between the top of your front axle tube and the bumpstops. My 2-1/2" lift springs (which I'm sure have settled a bit, but just for reference) have exactly 4" between the bottom of the bumpstop and the top of the axle.
