To find out if a constraint is binding you just need to look at equation_name.slack.

Assume you have that constraint:

e(i,j).. ... =l= ...;

Now you solve and afterwards you could selectively write only those instances of constraint e, which are binding:

file fx /binding.txt/; put fx; 
loop((i,j)$(e.slack(i,j)<1e-6), 
     put i.tl, j.tl, e.slack(i,j) /
     );