The most common types of standing rigging failure are either from broken strands in the wire itself, usually right at the end of a terminal, or a split in the terminal shaft itself. Broken strands ("meathooks") and heavy rust are obvious to sight and touch, but cracked terminals are often difficult to see. My spies tell me that they usually come from a salt buildup in the wire, inside the terminal, resulting in rust which swells up the wire and eventually builds up enough pressure to split the terminal. Upon inspection, if you see any hint of a split in the terminal shaft, or even heavy rust in the wire at the end of the terminal, replace it.
Two more things to watch for are kinks in the wire and bent studs or toggles in the turnbuckle. If the stud was bent enough so that after straightening the turnbuckle body won't turn easily it shouldn't be trusted. And don't push your luck by straightening a terminal more than once. Stainless work-hardens significantly and weakens each time it is bend and straightened. Same thing with kinks in the wire. If it straightens OK (again, only once) and the wire is still smooth it's probably fine, but the choice is yours. Use some common sense; a Montgomery 15 uses the same size of wire (1/8" 1X19) as a Catalina 22, or a MacGregor 26. On the 15 it's an overkill, but I would be much less forgiving in an application where it is more highly stressed, like a headstay on a 2,500 lb. boat.
We use open-barrel bronze turnbuckles (chromed) and they will last for many years if they are unbent and uncorroded, and cleaned up and lubed once a year they should last as long as the boat. Most modern ones have stainless studs on top and bottom, and stainless moves against bronze very easily without galling as long as everything is clean and lubed. When cleaning, take them apart and look inside at the threads. I have seen the threads damaged by someone running them up on bent studs and by burred threads on the studs. Turnbuckle bodies are expensive but if there are signs of damage, replace them. The downside of a failure is expensive and can be hazardous to your health.
When ordering rigging from me, you can inspect and measure it yourself and save the freight charges, or you can send me the whole thing and I'll take a good look at it, go over the terminals with a magnifying glass, and give you an opinion based on what I would do if it were mine. If you send me the specs, remember to verify that the top turnbuckle studs (the ones swedged to the wires) are right-handed; not left-handed. I stock right-handed studs but might have to special-order them if they are left-handed.
Read the page on Standing Rigging- Measuring for some comments on replacing old rigging with more up-to-date terminals.