^^
It really does depend, If the system is designed properly and does a good job then just fire it up and see if everything is operational. If everything is busted to crap and you have to put new lines in you won't find anyone that will want to make use of your old stuff so a new system is probably what you would be looking at.
First step I would do is to turn on the water to the outside source, you should hear an initial rush of water to charge the mainline but if it continues to run turn it off and try to locate the valve box with valves and manifold. Make sure the solinoids are hand tight on the top of the valve and any bleed screws are tight, levers closed etc. (you can turn a valve on manually by loosening the solinoid on top). If they are hand tight and the water is running look for any leaks around the manifold or anywhere that the mainline runs to the manifold if you cant see anything you may just need a new diaphragm in the valve (easiest way to tell this is if any of the sprinkler zones are running).
If the water stops running after charging the mainline again go to the valves and manually test them one at a time. This will make it easy to see if you have any leaks in any of the zones and also to locate the heads and what kind of coverage you have.
Like I said, if you are happy with the coverage and everything looks like its operational and working ok I wouldn't spend any money at all. Its relatively easy to check this stuff without paying someone but if you feel uncomfortable doing it then contact an installer.
Sorry if this seems a little scattered, I'm trying to play out what I would do in my head as I respond.
