I'm not sure that OIL filled Koni's come back up themselves.
But GAS filled shocks which most manufacturers use - whether cheap PhilJ or expnsive KW do come back up.
Its just Koni are different and often use oil (well my Koni yellows do), which is why in winter / cold weather shocks can still be soft, unlike gas shocks that are noticeably softer in the summer / harder in winter.
I imagine you could tell condition by slower pushing rod down, and pulling up and seeing how smooth it is.
Joe