GIT get the commit hash prior to a specific commit
Use git show HEAD^1. You can replace HEAD with your commit-hash
Edit to take multiple parents into account:
In case you want to see all the parents for a commit hash, you can use git rev-list --parents -n 1 <commithash> or use git show as @Bhaskar suggested in the comments to the question.
There are other ways as well as explained here.