Underwear or swimming trunk?

As the title suggest..
How do you determine if someone is wearing a
swimming trunk or an underwear?
Especially since both are triangular, and say, in same color?

In this instance, i say.. It depends..
Let's put aside the material used.
For a proper swimming trunk is made of lycra or similar material,
and is water and chlorine resistance..
An underwear, is normally not.

If someone wears a swimming trunk
and appear in a shopping center or restaurants, even if it is near
a beach or swimming pool, it will appear as an underwear.
And if someone wears an underwear in a swimming pool, then it will most likely be viewed as a swimming trunk. Or sort of...

Andway, my point is this, there is really a thin red line
between what people perceives. The context must be revelant
to determine the fine line between trunk and underwear...

Why, you may ask, am i talking so much about this swimming trunk issue..
It's because when i was swimming yesterday, i saw a guy beside me,
wearing an underwear to swim.

Not those black color one.
It's those thin white ones..
Where one can clearly see thru what's beneathe...

This my friend,
is unacceptable...