I just use the push and release test...
Without the spring on the shock, I hold the two ends and count as I fully compress the shock, then I count as it fully extends.
I realize that some of us are stronger than others, but a brand new shock for me compresses in 8 seconds and returns to full extension in another 8 seconds.
I have found that shocks will extend slower as they age. I had one of the old chrome shock, which come as spare parts, that compressed in 8 sec and extended in 17 seconds, which needless to say is no longer legal.
For purposes of identifying which corner to put the shocks on, this method should be good enough. Just do all your shocks at the same time. If you do them on different days or when you have "beer muscles", it may throw off the results.