1st off we're talking auto tranny right? there is a dipstick (forget which side of the engine is on) in the engine compartment for the tranny, its in a long tube that goes under the firewall. with the engine running pull the stick out clean it off then insert and pull out again, look on the stick and there is a range indication letting you know if its low, in range or over full.
if the fluid is brown or black have it changed or even flushed, smell it too, if it smells burnt you've got problems