Q: Has anyone heard of putting fibre optic camera into iphone4 to check for damage

No. Sounds expensive.

Out of curiosity: how/where are you planning on inserting the probe into the iPhone without taking it apart?

They are checking for water damage…

iPhone and iPod: Liquid damage is not covered by warranty - Apple Support

What do you think you could see if you managed to snake an endoscope into the device? There are almost no moving parts and the ones that do move are mostly switches that are visible from the outside. The vibrator is kinda obvious when it fails.

The gyroscope technically moves, but that is encased inside a chip.

Some nice images examples at https://www.ifixit.com/Teardown/iPhone+4+Gyroscope+Teardown/3156

Just as I thought... an expensive Bench Tool - Apple hands-on tech benches are really decked out!

I wouldn't even think about it if it's still under warranty. Or, for that matter, if the warranty has expired. They can tell if anyone has done anything besides using it as intended. And they will not help you with anything if you've tampered with it.

Yes, see the link.

iPhone and iPod: Liquid damage is not covered by warranty - Apple Support

It shows you the locations of the 'Liquid Contact Indicators'. It seems to me that some iPhones have one in other orifices too. The lightning port indicators seem to be on the 5 too for example.

You don't need an endoscope to see them. A good pair of eyes will see them with enough light, for everyone else a jewellers loupe & torch or bright light will show them.

What exactly are you looking for?