With Git, how can I tell if one commit in my branch is a descendant of another commit?
If you want to check this programmatically (e.g. in script), you can check if
git merge-base A B is equal to
git rev-parse --verify A (then A is reachable from B), or if it is
git rev-parse --verify B (then B is reachable from A).
git rev-parse is here needed to convert from commit name to commit SHA-1 / commit id.
git rev-list like in VonC answer is also possibility.
If one of commits you are asking about is a branch tip, then
git branch --contains <commit> or
git branch --merged <commit> might be better non-programmatic solution.