How can you tell if a guy is just shy?

I mean, this one was married for 4 years and with her for 10, he had gfs in uni but he's been single for ages. It seemed like he used to be really confident but he's a different person now. He doesn't comment on girls pics online, just likes them but has no issue having a debate with folk. He hasn't followed anyone on insta in ages
How can you tell if a guy is just shy?
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