How to tell when the gasket needs to be replaced?

What is the signs that the gasket needs to be replaced?
My glass stays clean in the top and center ( whine glass shape ) the rest gets covered with soot!
What causes this?
Thanks
Sean

It may be caused by air infiltration around the leaking parts of gasket. If you search for the "dollar bill test" you will find several discussions. Basically you take a dollar bill and close the stove door on it in several places and try to pull the bill back out. If you can pull it out easily, its time to replace the gasket.

take a lit candlestick, and when the fire is going good with the draft shut down, go around the door with that lit candle. You'll be able to see the flame "pull" into the areas where the gasket is leaking......
You can also do the smoke test in the same manner....take an incense stick and do the same procedure with it as the candle...that'll tattle-tale the leaks. Maybe have a headlamp or flashlight there so you can really see the whisps of smoke getting sucked into the leaky areas....

If you have any doubt, change it out.

When your shirt is on fire, time to replace gasket

Do what Scotty said it works well ! If that finds air leaks adjust the latch if you can before changing out the gasket that may be all it needs.

Pete

Thanks to all.
no problems just a question with good info noted!
Sean