It should be written on the box -- assuming you have the original psu cube. If it's the 12 volt model these are seemingly interchangeable with those used by Netgear. Any flea market/car boot sale should yield a suitable replacement -- even if you have to buy the router as well, it'll still cost less than buying an original Linksys adapter.
Thanks for the advice. The unit was given to me w/out packaging or documentation.