https://www.justice.gov/opa/press-release/file/1092091/downl... has a bunch of evidence that the Justice Department collected when charging some people associated with ATP38 around the Sony Pictures and WannaCry hacks (and other campaigns)
I'd note that things like shared encryption keys and shared TLS passive tables are very indicative of shared resources.
The use of North Korean IP addresses is indicative, but never enough on its own. However, the use of domains controlled by North Korean IP addresses is interesting as well.
Combine that with passwords largely shared with another North Korean attack, devices signed into from NK IP addresses under multiple accounts setup from N Korean IP addresses you start seeing a pattern of behaviour.
And then you find that the person who controlled accounts used by these attacks was a North Korean national (pg 134) who worked for a well known North Korean front company (paragraph 269, pg 136) and the evidence becomes pretty good.
Evidence is evidence. After WMD (which totally took me in, btw, you too?) Claims that evidence is "just over there" and "here are multiple different people reporting they've spoken to someone who saw it." Count for zero. Maybe they always should have but there's not doubt this stuff happens anymore. We watched it. (Hopefully) in horror as it unfolded without us objecting.
OTOH, the evidence linking APT38 to North Korea is pretty compelling. For example, there is a bunch of evidence collected independently identifying individuals associated with APT38, and these people worked for the North Korean company Chosun Expo.
See https://www.justice.gov/opa/press-release/file/1092091/downl... for the evidence in depth.
For what it's worth, quite a few people were skeptical about the WMD "evidence" at the time, and even more cynics like myself figured that true or false, it was mostly an excuse for George W to Do Something(tm) after 9/11 and at the same time finish off the war his dad started.