chain service limit

[bikeb4cage]

Is there a sure fire way to know if a chain is spent?  I know the OEM manuals will quote a 21 pin measurement (I.E. 319 MM for a 530 chain), beyond which a chain is "at its service limit" and needs to be replaced".  However, I can't seem to validate if the same would apply for an aftermarket chain (DID, EK).  I would think it would...however, I wonder - here is why:
 
I replaced my chain with an EK chain a couple years ago.  Have put 10,000 miles on it.  Have also cleaned and lubed religiously.  I measured the 21 pin length yesterday and get 318 MM (though, measuring with a tape measure, so not that accurate).  I also pulled the chain off the sprocket (or tried too, it was pretty tight), at the rear, to see if it would lift off the teeth.  I would expect an aftermarket chain to go upwards of 15-20K miles, hence my question.  The one wrinkle - I have a 14T countershaft sprocket (vs. stock 15T), and this would tend to accelerate wear.
 
Is there anyway to know if the chain is at it's service limit (beyond the adage that, if you have to constantly adjust the slack, then you are nearing the end)....what I am trying to do is determine whether to replace the chain before a big road trip next week or run it another 5K-10K miles
 
by the way, the chain slack adjuster (hash marks) is halfway back on the swing arm

[bikeb4cage]

The stretch limit for a sealed chain is 1%.  For 21 pins, this is about 3.16 mm.  I need to check my measurements (would be good to do with a vernier...but who keeps one of those around the house), because if I am off by just a mm, there is still 1/3 life left.  I think the real proof is where the rear axle is relative to the hash marks - I would expect the axle to be towards the end of the hash marks, when the chain is worn.  Not in the middle of the marks where they are now.  So, I think I am just being paranoid...and the chain is fine for a while.

[lucas]

My OEM chain lasted about 15k miles. But I should have replaced it sooner than that.

The way I knew for sure it was worn out was that it had tight and lose sections in the chain. I would adjust it and get it just right and then roll the bike forward a few feet and dam if the chain was then too lose and then roll it forward again and it was too tight. It was streched in some segments and not others. I have a DID now that has about 4k miles on it. It seems to be doing well. I hope to get 20k good miles out of it.

Lucas