Where is the check plug for the transmission on a mercury topaz standard transmission

Raise the car and check underneath. You should find a drain plug at the bottom of the tranny, and the fill plug is either on one side or towards the back. I can't find the diagram for your car, but this should be easy to find.
Some cars have a rubber check/fill plug. This is where you check the fluid level, on a flat surface. You also fill it up here, to the top of the check window. You have to use a fluid pump to fill it up. You can use a small hand pump that pumps it out of the container, or get fancy with a bigger model that holds the fluid also.
Some cars have a window cap that is a bolt with a half inch square in the middle. Just use a half-inch socket wrench with out a socket on it, or half-inch breaker bar.
Hope this helps.