Approved for tips, although I'm not sure this is an entirely reliable method for doing what the thread topic suggests.
It can be a fun activity to do with your tulpa, but will it truly convince someone of their tulpa's sentience unless their imagination is somewhat static unless the tulpa does their best to do so?
Something I found to work a bit better is to let my tulpa be naturally surprising/unexpected by observing them without influencing them:
1. Start narrating to the tulpa while passively focusing on their essence (that unique feeling every person has) and keeping a very passive focus on your imagination.
2. Just keep on talking and focusing on your thoughts (you could also focus on the sound of your inner voice).
3. Tulpa responds in all kinds of partially involuntary/unexpected ways: emotional responses/feels, thoughts, voice, that-unique-feel-you-get-when-another-person-is-sharing-your-head-with-you, ...
4. Were you doubting your tulpa's sentience before? What about now when you can't really prevent their replies (unless you purposefully try ignoring them) and especially when their replies make sense/are consistent and you didn't consider or predict them?
Getting to believe your tulpa is sentient is something that should happen with evidence (given to you by your tulpa - which will happen as long as you give them some chances - by paying attention to them) and should usually be a gradual process, not a belief that's forced overnight.