UV sterilizer question

(yes I know some of you don't believe in them...........vbg......... so
ignore)

do any of you KNOW where there is info on how you can tell if the UV filter
is working?
mine is in its 2nd season, has been running a month & the water is still
nasty green.

I am sure it worked OK last year ( at least water was pretty clear)
the UV filter is a 40 watt, for a 6000 gal pond ( MINE IS 3800 gal) & the
bulb is supposed to last 9000 hours.(pump is NOT that suctiony (g), so the
flow rate is not too high, its divided off of a swimming pool filter & the
UV filter is right before a bio filter.

the lights are still glowing on the thing ( its by pondmaster).
I do have a prefilter before it, to catch gunk & stuff & not let fishes get
sucked up, & water does flow through it!, even with the pre-filter the algae
should get into it to be killed right? not just stuck from going into the
filter, by that, so it just keeps floating around IN the pond?.

oh lessee
ammonia is 0
nitrite is 0
KH is 105
PH is 82 ( taken at late evening)
oxygen is 10 ppm??( is that the measurement)

been adding lymozine weekly & just added koi clay last week.