how can I find out the "source" changelist of the newly created branch in perforce?

To see details on source of branch integrated in perforce

After branching from base branch to a new branch, can we find the changelist at which the branch is created from source branch?

You can find it by using the below command with the new branch ( the child branch for whom you are interested to see the history)

p4 changes -i  //depot/child_branch


This will list  the  changes of this branch followed by the changes happened in its parent branch. By describing the changes you can get the particular parent branch name.

Syntax for p4 describe

p4 describe <changelist number>

example-
$ p4 describe 1129671
Change 7129671 by ravis1@project on 2010/06/29 06:51:50

        starting tomcat using "start startup.bat" command

Affected files ...

... //dev/env/atia/eventrepository/install/install.bat#11 edit
... //dev/env/atia/eventrepository/readme.txt#6 edit