Not always correct, but many medium and big truck 5 speeds have a normal shift pattern when they are direct drive, and the overdrive version will have a slightly messed up shift pattern. the last shift is usually a "U" or an "n" shift, and not a straight across shift like the first few gears are.
A tag with model number is the best way to figure out what you have. Casting numbers are the worst way, as the same case can usualy be used for OD and non OD transmissions.
Grigg
