Two months ago I got a little bonsai tree at a garden, a couple weeks later the leaves dried up. I didn’t water it as much as I should have unfortunately. I’ll leave a picture of it(I took it out of the pot and the soil fell off so I washed the rest of it off so I could examine it. How do I know if it’s dead and can I revive it?

Yes, I guess it is already dead.

Can I do anything to make sure?

You can scratch carefully the trunk and branches to see if they are green inside.
It's not a good idea taking the plant out from the pot at the moment, as you have done.
You can plant it on the ground, but I doubt if this will be of any help now, I think it's too late.

I replanted in a cleaned pot with a lot of drainage and the insides green

It has no roots, no leaves and is out of a pot. Any life has long gone. You cannot treat plants like that and expect them to live. If you learn from your mistakes you could now get another plant and know how to water it properly

The green inside means nothing?

BonsaiStarter wrote: The green inside means nothing?

Not really. If you see no green, it is dead. If you do see green, it doesn't tell much about it's future.
Not sure why you are calling this a bonsai, by the way.

I would not try to revive this myself. Even IF there still is life (The green under the bark means.. it is green. It requires a bit of experience to judge whether it is live, or just residual green). The way the plant looks, with shriveled branches makes me suspect it to be dead.

In some cases plants come back from looking like this. But on to of it, you removed all the soil from the roots. I do not expect this plant to come back. sorry.