'73 318 how do you know if original engine mount rubber is good?

They're really inexpensive, and the job isn't bad. A floor jack, block o wood, and think it's 5/8" maybe 11/16" on the nuts. A few of em are that "darn 1/8 th' of a turn" I did polys there and at the trans, also the rear, in my 74 Duster w a 340. It's a tight car...