Two things popped into my mind:

1) a simple thought experiment -- negate all things (the whole Universe/Multiverse including space-time and all) out of existence; if you are left with some abstract / concept, negate it out of existence too; what's left in the very end should answer your question; now try to negate that, that's where it gets really funny.

2) There's really no way around some kind of meditative practice, as in observing the nature of mind and thoughts and how they operate from subjective empirical perspective.

You might be interested in this piece I wrote some time ago (contains “spoilers” :D):

https://senderspike.medium.com/being-is-knowing-completeness-75db5c9c6349