When smoking salmon, how do I tell its done

[Captainslug]

I have read Kummok's recipe and I am going to try it.  But what I don't understand (and it is because I have never smoked salmon) how do you tell it's done?  Kummok's recipe says to "175° for 1-2 hours to finish"  Now with different thickness's and different temperatures people smoke at, I am assuming its not as simple as a duration of time.  How do you know that the salmon is done when you are smoking it? Is there a visual clue or is there an IT to cook to?  Thanks for all who answer.

[Habanero Smoker]

Most of the time I will take the salmon to an internal temperature of 140°F, but at times I want it a little drier and will continue to cook until it flakes.

[Captainslug]

Thanks Habs!

[aces-n-eights]

I use the taste test method - basically take a piece off the rack and see how it looks when i break it in half.  I like it a bit more "Done" than my wife, but that works out OK because, as you stated, there are differing thicknesses and some cook faster.

I see you have a fan... does that seem to help evening the temps out?  I have to rotate the racks and even then have some get done quite a bit faster than others.