Rear shock K75RT, When to replace?

There seem to 100's of tire inquiry threads that generate tons of opinions. Likewise, there are quite a few replacement shock threads but I can't find any that help me decide if I really need to replace the shock. So, as a new member and new rider, here goes my request:
I recently tackled and completed my first clutch spline and drive shaft spline lube, thanks to this forum. A little more reading on the Forum and I am wondering about the OEM shock on my 1995 K75RT, 41,000 miles. I never knew what it's life span should be and the various postings make me wonder what's the status of my bike's shock. Is there anyway I can effectively measure/gauge it? This is my first bike, as of last August, so I do not have the subjective ability that comes with experience. Along those lines, I am quickly learning that the motorcycle is probably a never ending repository of my cash. My attempts at DIY helps minimize that pain, but only slightly.
Assuming the answer is understandable by me and I determine the shock should be replaced, I've narrowed it down, from all the other discussions on performance, value, etc, etc to the Progressive line. Best prices I've found for the 412 Series is $207, 420 series is $410, and 465 series is $368. Any suggestions on why I should pick, or stay away from, any of these three?
Thanks in advance for any help thrown my way.

I'm far from any kind of expert on the subject but if your bike already handles and tracks fine loaded with rider + gear or passenger you're probably OK for now.

Having said that, almost everyone here will recommend replacing the stock shock and fork springs at some point, and Progressive certainly makes the list of recommendations.

Lots of aftermarket shock threads are on this site and if you use the Search tab located on the darker gray bar above you should find them (I think the main SEARCH tab near the top of the page doesn't always work)---perhaps one of them will better answer your question.

Thanks Jeff for the quick and helpful reply.
I believe it tracks and rides fine. But then again, it's the only bike I have ridden so maybe I will find a local expert to give it a try and add his opinion. Hate to add another $300-$500 to it if I really don't need to.
Thanks again and I will try the different search buttons. Wasn't aware of the difference.

Some folks will tell you the OEM shock is junk when bolted on at the factory. I don't agree and used OEM shocks several times on my K75. I did use the "S" shocks which were in theory at least different than those used on the C, T, and later RTs with the long travel front forks. I also converted the forks to the S suspension which made a better handler with less cushy fork travel but slightly rougher ride on small bumps.

I have used OEM, Works, Progressive, Nivomat (yes, Nivomat without a spring) and Hagon on K75s. From among the shocks you listed I would use the Progressive 420. To me the internal nitrogen bladder is worth it. YMMV

sleeper said:

This is my first bike, as of last August, so I do not have the subjective ability that comes with experience.

I believe it tracks and rides fine.

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You are probably fine with the shock you have.

You will always have the option of upgrading your suspension when your skills develop to the point that you actually need the upgrade.

My '95 K75RT shock has 60k miles on it and it is fine for everyday riding. I switched to a YSS shock for a bump in performance. I tried the Progressive 420 and found it to be too harsh.

If you really have a 1975 K bike

you definately need a new rear shock.

It might help to mention signs of a failing rear shock. Personally, I knew the rear shock on my '88 K75S was going south when it started to "pogo" in the back when stopping abruptly. That, and in tight turns it started to feel like it was wallowing a bit.

Shocking K75RT

Just put a new YSS on my 95 RT. It has made it a new bike! No really! It's that big a difference. Much more solid feel and handling and half the cost of a Wilbers. I used to get a little mushy feeling when coming out of a set of sweepers. Now it's rock solid.

+1 on the YSS shock. It will transform your riding experience!