GMC Sierra 2000 Owner's Manual page 294

Then, without shutting off the engine, follow these steps:
1. The red transmission dipstick handle is located at the
rear of the engine compartment, on the passenger's
side. Flip the handle up and then pull out the dipstick
and wipe it with a clean rag or paper towel.
2. Push it back in all the way, wait three seconds and
then pull it back out again.
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3. Check both sides of the dipstick, and read the lower
level. The fluid level must be in the COLD area,
-
below the cross hatched area, for a cold check or in
the HOT area or cross
4. If the fluid level is in the acceptable range, push the
dipstick back in all the way; then flip the handle
down to lock the dipstick in place.
-
hatched area for a hot check.