Typically the ones on the top of the engine fail first. Typically, a vacuum line fail will eventually get bad enough that it will trigger a fuel trim related diagnostic trouble code. At least that's what happened on my AEB. The small vacuum hose can be purchased for relatively cheap. I recommend staying away from the fabric braided OEM stuff. It is too expensive and it doesn't last long, especially in dry/hot environments. I had to replace my first set of 3mm vacuum lines at the two year mark. I've got silicone hose on there now.
If your car is out of warranty, just start replacing the stock braided vaccum lines (if that is what you have). You could spend a lot of time and aggrevation on diagnostics, but vaccum line is cheap, and you are going to need to do it sooner or later anyway.
A forum search should turn up several sources for quality line.