early CJ-5s had a stamped VIN on the passenger side frame rail directly below the firewall, or in the rear above the shock mount like RenegadeT described. It is possible that the frame rails are not in such great shape so you might not find it. The V6, if original, narrows it down to 66-71. If the rear axle is an offset D44 with one piece axles, that narows it down further to 70-72, with 71 being the most likely (again, if the axle is original). If you are just trying to find out so you know what parts will fit, then just look at the leaf springs like mentioned earlier. If they're narrow, you have an early CJ-5 (55-75), if they are 2.5", then you have a late CJ-5 (76-83). Parts between those years are interchangeable.