how can we come to know that iphone is hacked or not

All such messages are scams from criminals, 100% of the time.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Click here→Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

dharshan256 Said:

"how can we come to know that iphone is hacked or not: I think my phone got hanged and i got a error while browsing firefox and i want to know whether it is hacked or what"

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Report a Scam:

Was this error you speak of a a text message? If a text message - it's a scam. So, take a screenshot of it, and report it to Apple. Learn more here: Using the Photos App to Report Scams - User Tips

dharshan256 Said:

"No it’s not a text message its like a pop up i got when I opened firefox"

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Screenshots:

The next time you get such as thing as that, take a screenshot, and send it to Apple as an attachment. Send it to: reportphishing@apple.com, and explain what this attachment is all about: a scam message.

If you never jailbroke your phone it isn’t hacked.

Because iPhones can’t be hacked.