This is a follow-up to my answer and the corresponding question: https://meta.askubuntu.com/a/17148/420786. What is a viable alternative to apps.ubuntu.com?
I personally typically refer to Ubuntu Packages Search located at https://packages.ubuntu.com/ as mentioned by Mark Kirby in this comment Thus far I've found the information to be up to date covering all supported versions of Ubuntu. By crafting the search properly it can also help future proof your answer as shown in this ffmpeg search.
The inclusion of suite=all in the search returns results for all currently supported versions of Ubuntu. Of course you can specify the release if the question you are answering is version specific as in suite=xenial for 16.04, etc.
apt installcommand or just the name of the program which can be found in the software center?