You can easily check to see if the basics work... Put both shift levers in the middle position and you will be in neutral. Should be able to turn input shaft while holding output shaft from spinning. Move the shift lever in the front either direction to test 3rd & 4th gear. The "middle" shift lever runs 1st & 2nd. and the rear lever is for reverse. When moving any of the levers, be sure the remaining levers are in their neutral position. You can test each gear to be sure the output shaft turns accordingly.
It may take more of a trained eye to determine the condition of synchros, bearings, etc. I recently rebuilt my muncie and stuck about $600 in parts into it. The main reason I opened it up was to replace 3rd gear, but also ended up replacing the Counter gear/shaft and the input shaft & gear. Meanwhile, mine would have looked plenty good on the bench, as these issues would not have been known until really digging in.
Used units can be a gamble with no warranty behind it. I think you can get fully rebuilt Muncies four around $1,000. I used Riverside Gear for my parts and general info, along with a $20 manual. They also sell complete trannys.