Elastic power indicators tend to be a decent indicator of eccentric strength abilities. As a result you could compare static lifts with counter-movement lifts or static jumps with counter-movement jumps. Similarly you can generally get an idea by seeing if there is a big disparity between weight room performances and speed performances.
As for when to work it, I work it practically all year round in some capacity. Plyos are probably the most commonplace form of eccentric / elastic strength development. Slow eccentrics can be a useful training tool but IMO should really be secondary to faster eccentric work or even traditional tempo (not tempo running but tempo as in speed of movement) training.
BTW, when I think of eccentric abilities, I'm not speaking of slow eccentric strength as it is commonly used in body building circles.
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