How to know if my device has been reported lost or stolen

if you found an iphone then you would not be able to bypass it's activation lock

Activation Lock for iPhone, iPad and iPod touch – Apple Support (UK)

If you found an iphone, you are obligated to turn it in to the proper authorities. Failure to do so may constitute a crime commonly known as ‘theft by finding’.

I'm confused by your question. Did you find YOUR lost iPhone or did you find a lost iPhone, which you would like to use?

If you found a lost iPhone, which was not your iPhone and was someone else's, turn it over to the authorities as you simply cannot use it.