How do you know when your transmission is going?

The best way to tell is by checking the fluid. Check the color and odor of the fluid. bright red is OK ,and should smell like unused fluid. A brown color with a burnt smell indicates wear, no good. Drop the pan and see if there are metal filings in the fluid, no good. Slippage can just be an indicator of a band adjustment, and noises that you think are coming from your transmission can actually be coming from your differentials, u - joints, transfer case ..etc. In other Words, sounds can be transmitted from one point to another, like putting your ear on a train track when the train is miles away...