well one way to tell is If you put the transmission in the top gear and spin the input shaft... the output shaft should be spinning faster. it may be eazier to see if you mark both of them with a grease pen. if you can give us more info as to what year truck you are talking about we may be able to narrow down the transmission for you. there were a lot of clark transmissions used in the early days... some being overdrive, some being direct.