Useful links:

• https://bugs.debian.org/release-critical/other/testing.html - Shows the number of release critical bugs that are affecting testing
• https://bugs.debian.org/release-critical/ - Shows how many release critical are there that affect next release
• https://www.debian.org/releases/testing/ - contains a link to release-critical bugs

Use

lsb_release -cs

Sample run

 % lsb_release -cs
buster

Ref:- I came across it in https://www.texmacs.org/tmweb/download/linux-repos.en.html#debian