How can I ask via formula if a button is active or not?

As @Kristofer_Tripp said, the best approach would be to calculate if the button should be enabled/disabled as a separate column, then both filter by that value and use that value in “Disable if” formula of a button, inverted.

However, for answer completeness — yes, you can filter out the buttons that are disabled, i.e. whose action is internally set to _Noop(), with a formula like this:

Find('"name":"_Noop"', _Merge(thisRow.[My button]) + "") = -1

Warning: this is a discouraged approach. I only give it here for flexing illustration purposes and giving you an idea of the power of Coda.