I think it's partly experience. The more garments you have handled, the more you easily you can tell.If styles are repeated in different decades, for example 40s styles in the 80s, how do you tell? I am baffled by how some here were able to just look at my blouses and tell they are 80s. It's kind of making me question everything, LOL.
But I wasn't sure about your first blouse - I did think it was later, but I wasn't sure. That one for me was mostly about the print. It just didn't quite look like a 40s floral to me, and I'm not even quite sure how I know that. The buttons might be older than 80s, but they don't really look as old as 40s. Of course it's hard to tell much by buttons, because they are so easy to replace.
Similarly the grey leaf print didn't looks 40s, although I'd have wanted a close up to be sure. The lace one, is quite simple in construction, 40s clothes tend to be more detailed, eg more darts etc. Not a hard and fast rule of course.
And the fabric and print on the blue blouse, looks similar to things that I know were 80s.
Also, if you've lived through a decade (the 80s), you kind of recognise clothes you might have had!
Another 'rule of thumb' for me is that if I look at something and think 'is it 40s or 80s?' I remind myself it's more likely to be 80s, because there's just so much more 80s stuff around! So I start from 80s, and only if I find something that really screams 40s, would I assume it's older.