How do you determine male gynecomastia?

I didn't know there was such a thing as male gynecomastia~

My husband has a slim build, but as he gets older, his belly slightly protrudes.

So my husband started exercising~ and since then, I feel like his chest has slightly become more prominent.

My husband insists it's muscle~~

After hearing this, I wonder if it's gynecomastia in men~? It makes me suspicious ㅠ

Is there any basis for us to make a judgment?