Regarding the original intent of the gear:
The red harness appears to only have a dorsal D-ring - this is suitable only for fall arrest.
The yellow harness appears to have a dorsal D-ring as well as shoulder rings - this can be used for vertical Confined Space Entry (CSE). For CSE, the dorsal ring is for fall protection when entering and exiting the space (attached to a tripod or other elevated anchor) and the shoulder rings are for rescue from the space. By using the shoulder rings, the body remains more vertical for easy extrication through the (usually small) hole, as opposed to drooped forward as caused by the dorsal ring.
Neither harness is suitable for tree climbing. They lack the requisite positioning D-rings or the central anchor point at the hips.
Two lanyards are "pig-tail" or "split-tail" lanyards used for 100% tie-off. Usually these are used on a continuous steel wire rope fall protection system that has intermediate anchors. Similar to passing a limb on a tree using ALT, the user can pass an anchor by clipping in beyond the anchor, then unclipping the other. They both have stitched deceleration packs that are designed to limit the maximum arresting force (MAF) on the body to about 900#.
The other lanyard is a single with a deceleration section built into the lanyard webbing itself. This would normally be used for tie-off in a manlift or bucket truck where only one lanyard is needed.
The advice to destroy them is certainly the safest given the unknown history. That said, a detailed visual inspection by a knowledgable person should reveal any critical defects or damage. If it hasn't sustained a fall, the main damage that couldn't be detected by a visual inspection would be some sort of chemical damage (weakening on a microscopic level). Although most workers have a personally-"owned" harness, at my work, our equipment cache is shared by many occasional users. Therefore, the exact history is never known. The gear just receives a detailed visual inspection when it returns and before it goes out the next time. If our gear actually had taken a fall, it would be taken out of service regardless of whether it was observably damaged or not. Of course, unless it's a serious fall that requires rescue and becomes an event reported to OSHA, there's the very real possibility that it'll never be reported to anyone. This is usually due to fear of embarassment and discipline. If it were investigated by OSHA, I'd imagine that they'd seize the harness as evidence in their investigation.
I have an agreement with the equipment manager whereby he gives me the out-of-service lanyards and harnesses so I can salvage the hardware for personal use. Usually they've been contaminated or the soft parts have been cut, frayed or otherwise damaged. I've never done anything with the harness hardware, but I probably have a dozen double-locking steel snaps, which are very nice to have. This is what I would recommend.
