How would you know if the Chevy th350 transmission pump goes bad?

If a th350 pump goes bad. First thing to check is fluid level, first with the eng not running the level should be high on the stick. Then start the engine and quickly check fluid level again, if the level does not change pump may be suspect. If the vehicle moves at first then stops moving the filter may be clogged (a definite internal trans problem indicator) if a trans pressure gauge is avail ,test the port @ rear of pass side of case behind the round cover