determining specific activity

[Hip Tripper]

How does one determine the specific activity of a labeled sample? 
Does anyone do this?

I did an amino-allyl labeling and the resulting conjugation to
Amersham's mono-functional yielded what subjectively looked great
(very dark purple following multiple washes).  However the S/N was
very low and I was not able to get any data from the hyb.  I would
like a means of assessing the sample prior to hyb'ing.

Thanks in advance!