It is very hard to POP adult snakes, if at all. Spurs are a good indicator but the sure fire way is probing. Be gentle & slow with this or get an experienced person to do it & use LUBRICANT.
i have had dozens of snakes of all kinds that i was unsure about, i can usually tell by the tail after 37 years of herping but Rosies can be tricky.
Take your time, be slow & gentle. I often use an eye lens to check out the spurs before i probe. I cant remember the last time i was wrong, having said that it does take time & practice.
Good luck & happy herping.