On/Off rates of Rec. Fabs?

[Mxm]

I have produced a set of recombinant Fabs using Phage display.  Now I was
wondering if there was a way to screen them for On/Off rates.  Must I
purify the Fabs from the bacterial supernatents in order to get this
information?
We are planning on subcloning many of the clones into a Histidine tagged
vector for Ni+NTA column purification.  I would like to avoid having to
subclone all of them since there are so many positives.  I would prefer to
pick just a few which have the required characteristics.  Any suggestions
or sources of literature on techniques would be greatly apreciated.  Feel
free to respond on my E-mail if you prefer.
   Peter