https://blog.cryptographyengineering.com/2024/08/25/telegram...
Ultimately the only way to be completely sure is to use an open-source app like Signal that you've either built yourself from source you've inspected; or sourced pre-built from someone you trust.
If neither of you have done this, don't worry the client side code is so sloppy there will be a zero click RCE that can steal all your chats anyway.
Telegram also has no public security or cryptographic assessments, while meanwhile WhatsApp has had numerous components analyzed by cryptographers for security.
https://www.nccgroup.com/research-blog/public-report-whatsap... https://www.nccgroup.com/research-blog/public-report-whatsap... https://www.nccgroup.com/research-blog/public-report-whatsap... https://www.nccgroup.com/research-blog/public-report-whatsap... https://www.nccgroup.com/research-blog/public-report-meta-wh...