There is an option in the contacts to "merge" SIM contacts to Google. I would do this first, just to be sure that you've gotten them all. You may end up with duplicates, but IMO that's better than losing some.
Once you've done this, I would take ONE contact that's on my SIM (jot down the information on paper "just in case") and delete it from the SIM.
Wait a few minutes to allow the regular syncing to take place, then check your contacts and see if they're still there. If they are, then you're good. If you want a triple check, go to "contacts.google.com" on a computer and see if they're still there. They should be.
You would then be safe to delete everyone from your SIM contacts because they're already backed up.