Unfortunately, I'm not aware of any easily available test for juglone or tannin accumulation in pond water (or even in soil where it's more commonly an issue).  Juglone is broadly toxic, including to fishes, but I have no idea how much leaf or husk material (nut hulls are much more toxic than leaves) would be necessary to randomly drop into a pond before stress becomes evident.  Most people don’t tolerate substantial canopies of deciduous trees in general near to ponds because the accumulation of organic matter from excessive leaf litter tends to hasten the aging process of the pond.  Conventional wisdom is certainly to keep walnut trees at a distance that they’re not likely to shed into ponds, but I can’t confirm any associated quantitative details.

Leaf litter itself will impose problems on a pond.  Increasing the organic matter decomposing in the deep water of your pond will, again, hasten the aging process and increase the biological oxygen demand of the system with it.  That can lead to wide fluctuations of dissolved oxygen, concentrations being lowest after the length of night, just before dawn.  Low-oxygen stress is easily the most common cause of fish kills on small ponds.

You can help slow the pond's aging process and mitigate low-oxygen events by operating diffuser aeration from deep water throughout the warm months.  However, you'd also be well served to simply manage against the accumulation of leaf litter and other organic matter in your pond.

I hope this information is useful.

Eugene