If you can’t tell … you have no business asking.
If you can’t tell … no amount of story-point estimation, rigorous sprint planning, measuring velocity, comparing team velocities, or daily check-ins will give you the answer.
Skilled developers know if their teammates are slacking. It’s there in the code, in the tests, and in the code reviews. Whether they share that information openly is another question.
If you are not a skilled developer, don’t bother trying to figure out if your team is slacking. Your data will be inconclusive. Work to create an environment where teammates 1) have a reason to be motivated, and 2) have the requisite safety to hold each other accountable for doing right by their fellow teammates.
Ask “what is holding us back from doing our best work” and pay attention. There’s often a story there. Build trust.
And/or learn to be a software developer.