How do you tell if your transmission is going bad?

Ummmm, it doesn't work right? Check the Transmission Fluid for proper level, smell and color. Fluid should be bright red, not brown and smell burnt.

We really need a whole lot of information here to answer this. What kind of vehicle? What is your transmission doing? A bad governor can make you think the thing is a total loss. It cost about $15.00 to fix on my GM. A vacuum hose, dirty filter, etc can all make it seem like you have a major problem. Now, if it slips in every gear, won't upshift or downshift, won't move in one direction or the other (or at all) you probably need serious help! Some of the imports have seriously sophisticated transmissions and only qualified techs can fix them.