Usually shrapnel, working out the depth at which it exploded, the nature of the explosion (recorded on video), and historical evidence of bombing.

There is a buch of forensic methods around this.

I understand that is what is usually used, but I was asking for the actual details which was scanty in the article. Is this what they did or are they still looking into the forensics?

Forensics identified a valid digital certificate matching ww2.co.jp. The issuing CA is currently denying responsibility.

It's pretty easy to identify from the shrapnel.

I understand that it is easy, but is this what they did? That was my whole question and nobody is actually answering my question

Because it's a silly question. The airport was bombed in WWII by the US, just like the rest of the country, and Japan wasn't in any war since then, nor was it in any war before then when it would have had such bombs dropped on this location (there was a naval war with Russia in the 1900s, but that didn't result in bombs dropped on Japan).

So basically you're asking for forensic evidence to prove that this bomb wasn't the result of time-travelers or teleporters or something equally fantastical. It's a ridiculous question. Of course it's an unexploded WWII bomb, what the hell else would it be?