How can I find the max u value of my NURBS curve?

The maxValue attribute on the shape node of your NURBS curve will tell you this.

import maya.cmds as mc
uVal = mc.getAttr('myCurveShape.maxValue')

You could then (for the sake of example) select that point on the curve via:

mc.select('myCurve.u[%s]'%uVal)

It should be noted that this length is in 'internal units' (cm), not necessarily what your 'ui units' are.