Currently I have .ipa file and same app can be installed through test flight. I don’t have the app source code. I tried extracting the files from .ipa file using Archive Utility but there was no plist file. I am not sure how do I get the bundle id for this app- (either from .ipa file or by using the installed app and what is the process?) as i want to automate this app test on real device
(Note: .app file or source code is not available and the test app is signed with valid developer provisioning profile)

If you have access to the testflight admin you can get the bundle id from the app information on that site. Otherwise you can extract the file using archive utility like you said and right click on the application within and select show package contents, this is where your info.plist will be stored.