Water resistant check?

I take out the sim card, and suck on it with my mouth... See if it's vacuum tight.

Hello,

You can't necessarily check the water resistance of your iPhone.

Here's an article:

About iPhone 7 and iPhone 7 Plus splash, water, and dust resistance - Apple Support

How to check if your iPhone has had previous water damage:

Liquid damage to iPhone or iPod isn't covered by warranty - Apple Support

Hi

It is not possible to check the water resistance of your iPhone.

If you are concerned, I suggest that you discuss the matter with the Apple Authorised Reseller from which you purchased the iPhone, at the same time as you discuss your other concerns, which you voiced in a different thread.

Also, please note from those links it is water resistant, not water proof. Water damage still voids your warranty, so you are wise to try and just avoid water exposure as much as you possible can (i.e. don't push your luck and think you can go swimming or showering with your iPhone).