It can't be proven.
To prove that something is infite you have to travel it, follow it forever, and maybe, just maybe, the creator of the universe built a big brick wall at the end of space, behind which is nothing, a space that cannot be entered.
Any human would have died of old age before it could be proven, so maybe if you sent a robot that was capable of functioning generation after generation, thousands of years.
And then capable of coming back again with the info.

But then if there is infinity he won't come back, and then we'll still think he's going, but maybe he just broke down....oh dear, I'm arguing with myself now.