The easiest way to check is to pull the driveshaft, put a driveshaft yoke into the rear of the trans, and spin it. It should only spin one way easily. If it spins both ways easily, the sprag is turned and it's got come apart.
Many sprag failures might seem to be sudden but they're usually preceded by flattening of the rollers. A sprag with several flattened rollers might pass the "turn it backwards" test (which is highly judgemental) but might still be in distress.
The only way to positively judge the condition of the sprag is a periodic visual inspection.
A lot of us old-timers used to abuse the sprag and never have a failure but there's a difference between doing it wrong and getting away with it and doing it right....I'm one of the lucky ones who did it wrong out of ignorance. No excuse in this day and age.
