No telling what its condition will be, from working just fine to total junk.
Can you get it freshwater for flush and saltwater to push through the filter? If so, divert the output into a jar and run it. Keep running it if
water comes out until it loses the hydrogen sulfide smell. If it's still pumping out, test what you've got.
RO watermakers are supposed to have all kinds of
maintenance. Yours has not. But then, my
watermaker doesn't seem to care - I neglect it, I start it up, it produces a liter or so of nasty stuff, and then it produces
water better than our municipal supply. You might get lucky.