I have a Sentry Valueguard safe that the combination is unknown. It is 8 1/2" round, stand 8 1/2" high and the dial increments go 0-100. How would I determine the combination.

I can usually crack simple left-right-left or right-left-right combination locks in about 20 minutes. Apparently, it's very difficult to find someone who can. I know of techniques that have been published on the net.

The particular method that I use, I developed on my own in high school.

I've opened combo suitcases on the first try, but that was an educated guess and I've done a few safes.

One, I opened in grade school while I was sitting on it.

This may help:
https://www.sentrysafe.com/customerCare/retrieveCombination.aspx
Personally I like to figure out myself, a challenge, like a puzzle.
It is amazing how vulnerable many locks are.

Strat:

Do you do combo cracking too? I forgot the combo to a small padlock (wheels) and I kept trying what I thought the combinations might be and got nowhere.

So, I said to myself, It will be easier to pick it, Sure enough, it was.

They are sometimes easier than rotary combo locks.

A co-worker said to me: Do you know how to bring the copier out of standby? I asked why and his answer made sense. I said I didn't know, but I would try.

I got it on the first try. It was an educated guess.

A number of people, including my boss asked if I could get into a particular room after hours, Again I asked why? If I was satisfied with the answer, they made a promise not to tell and I would not devulge how I did it. If I did, the security problem would be fixed and I didn't want that.