You should be able to. In XTR Rom, first do Settings->SD card & phone storage. If you see "Internal phone storage", available space of over, say, 90 to 100 MB (mine is 137 right now), you probably have a2sd.
In Root Explorer, go to the System folder, then the Sd folder in there (or, /system/sd), and you should see folders for app, app-private, dalvik-cache. If so, and there are apps in there, you have a2sd.