Looking for installed libraries/executables with XPInstall ?

Hi

i want to build an XPI for the Tcl Plugin 
(http://tclplugin.sourceforge.net).

Basically it should work like this:

(1) Check for a local Tcl installation, if found (3)
(2) Install Tclkit or ask the user to install Tcl
(3) Create local library folder for the plugin, copy files to their 
locations

The problem i have with XPInstall is, that i do not know how to look for 
a local Tcl installation from javascript.

To find a local installation i would try to execute some files on the 
local system:
tclsh, wish, tclkit, ...

File.execute() from XPInstall would do this only after i call 
performInstall(), which is to late for deciding what to do.

So, is there a way to execute some tests (like unix configure does) 
before i call performInstall()?

Michael