The description of the Cubic Gate (DMG, p. 160) states:
The six sides of the cube are each keyed to a different plane of existence, one of which is the Material Plane. The other sides are linked to planes determined by the DM.
I'm the DM in this situation, and I've decided which planes the cube is linked to. However, since the item does not require attunement, is there any way for the PC who now has the cube to determine those planes other than by experimentation?
(If the item did require attunement, I'd assume that attuning to it would let you know, since attuning to items allows you to understand its properties, but that's not the case here as it doesn't require attunement.)