I requested that my conservatory be built with argon filled Pilkington K glass. I can see that the window panes are Pilkington K but cannot see that they are argon filled. Is there any way I can tell that they are filled with argon gas? My neighbour says that I should be able to see a plug in each window pane where the gas has been inserted. Is this correct?

No

Thanks biffvernon for your explanation.

I think that you can prove if it in there or not by doing a refractive index test.

I asked the same question when we had the units for our windows. The only evidence of the Argon fill was on the sticky label on the glass, which of course you have to peel off if you want an unosbtructed view! I kept mine and stuck it to some old wooden stepladders (first thing to hand) so I know where the evidence is if I ever need it.
I've just had a look at the windows and I can't see anything from the inside of the window, but I do remember the black rubbery stuff on the edges when we got them. I guess it comes down to trusting your supplier.

Had a chat with a bloke who makes argon filled units regarding how it is done.
Aparently until recently there were two methods. The first was to have a hole in the bottom and one in the top. Argon is then blown in at one end and when the sensor at the other end detects Argon the filling is stopped and the two probes carefully extracted and filled with the afore mentioned black sticky stuff.
The second method I think involves removing a corner of the spacer, but no too sure how.
Quality of the work seems to have varied from producer to producer.
Aparently the latest technology involves the IGU's being sent into an argon filled chamber and sealed before they are removed. There are therfore no holes to fill in the edge spacer and hence no tell tale signs of filling.
Didnt have time to discuss more than the basics, but the newer method makes sense.

Way cool!