On Linux, you could read swap usage from the /proc filesystem:

% cat /proc/swaps
Filename                        Type            Size    Used    Priori
+ty
/dev/ide/host0/bus0/target1/lun0/part3 partition        2056312 0     
+  -1
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From this, a perl program:

open IN, "</proc/swaps" or die "No swap file: $!\n";
<IN>;
my @fields = split <IN>;
$field[3] = 0 unless $field[3];
print "swap used = $field[3]\n";
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-Mark