[Mr70]

No sure fire way,without docs.
And even their not rock solid today.
You have to look at a multiple of things.
Past owner History is a good start.

The rest of the drivetrain,if authentic,can speak volumes.
What is the Trans. type?
If it's a Manual,is it an M-22 & is the cars VIN# on it?
If it's an Auto.,is it a TH400?..whats the date-serial no.-and suffix code on it?

What is the Rearend Ratio,suffix-date-casting date as well?

Also,the time period it was produced & Assembly plant can lend some guidance.That can all be revealed in the full VIN#.

You can PM me these details if you like.