It should look for it from an environment variable.
For an example of this behaviour look Darcs.RemoteApply, used by darcs push too.

There are two different issues: Starting another instance of darcs on the local 
machine, and invoking darcs remotely via SSH.

Locally, we can use getProgName (as kirby pointed out, this would be helpful for 
Cygwin). Is it always fine to invoke darcs via ARGV[0] ?  

For remote invocations, Petr suggested to add a flag like --darcs-remote-
command, which should be filed as another bug.

See http://bugs.darcs.net/issue223 for the --remote-darcs request (which I think
anybody should feel free to take up as it's been assigned for a while, maybe
pinging CKeen)

The following patch updated the status of issue1377 to be resolved:

* resolve issue1377: getProgname for local darcs instances 
Ignore-this: d46f7d47fa663c8b68ab4a930ceb3e2e