Just in case your on Win32, here's how to obtain that information.

P:\test>perl -MWin32::SystemInfo CON
 Win32::SystemInfo::MemoryStatus(%m,'MB');
 printf '%15s := %15s'.$/, $_, $m{$_} for keys %m;
^Z
   TotalVirtual :=        2047.875
      TotalPage :=     501.4296875
      AvailPhys :=    112.84765625
      TotalPhys :=    223.23828125
        MemLoad :=               0
   AvailVirtual :=   2015.80859375
      AvailPage :=      308.515625
[download]

Simple enough that Abigail wouldn't need to give up his day job if charged with this onerous task:)

The significant value as far "swap space" is concerned is the last value, AvailPage. Unsurprisingly, this is the amount of page file space currently available for use. It's when this value begins to approach 0 that you will get the infamous "You are running low on system resources" popup.

Actually, it appears to warn you when this is approach around 15MB, but I haven't been able to confirm this. Of course, but the time you reach this point, things are getting pretty sluggish, as most of the OS itself has also be swapped out by then