A good and fast method would be with a color meter. There are devices that give you the green intensity of a leaf quite easily. Otherwise you could do it with digital imaging. But this means that you already have controls of how a senescent leaf looks like.

Other option but quite more time consuming would be some kind of enzymatic assay. But this method in itself is invasive so you would be damaging your leaves. You could also look into somethingike ethylene production.

Thanks Prof. Ioannis Baltzakis

You are very welcome. But I am not a Professor, just a simple PhD Candidate :)