You can also use an External Diff tool. For example I am using Beyond Compare. After you set it as an external diff tool in sourcetree, you select both files you want to compare using CTRL and after that you press CTRL+D. As a result, Beyond Compare will be launched with both files ready to be compared.
Another way to do this is to right-click on a branch and select the "Diff against current" context menu command (current refers to the branch you are currently working on). This will give you the diff between the head commits of the two branches.
You can also use an External Diff tool. For example I am using Beyond Compare. After you set it as an external diff tool in sourcetree, you select both files you want to compare using CTRL and after that you press CTRL+D. As a result, Beyond Compare will be launched with both files ready to be compared.
Use ⌘ (OSX) or CTRL (Windows and Linux) and choose any two commits you like in log view. It does not matter what branch the commits belong to.
As a result you will see something like...
...down below.
Another way to do this is to right-click on a branch and select the "Diff against current" context menu command (current refers to the branch you are currently working on). This will give you the diff between the head commits of the two branches.
Source: https://answers.atlassian.com/questions/167126/sourcetree-diff-against-current-behavior#
Screenshot (based on Sourcetree v1.9.5):
After choosing "Diff against Current" from the context menu, you should see a message to the effect of:
You can also do this by ctrl + clicking the two
branchescommits you want to diff.