While you can't find out when someone is on the app, there are ways to know when someone is online on Twitter.
How to Know if Someone is Viewing Your Twitter Account (2024)
Unfortunately, the answer is Yes/No; it's a two-way answer because you can't specifically tell if someone is online, but there are ways to tell. Some users are active on the platform, tweeting, commenting and liking every post they can.
These types of users are probably online; you can find this out by paying attention to their activities and when they perform them. You can check out how to do this in the later part of the article.
There is no direct indication of online presence, but you still have several options to find out if someone is online or not. But it would be best if you remember that these are just suggestions and not an insight from Twitter itself.