how will I know if my iphone is backed up

There are different ways of back up , if iPhone data is backed on Mac or you have enabled back with iCloud itself from iPhone see the details in these articles How to back up your iPhone, iPad, and iPod touch with your Mac - Apple Support

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

We don't know.

You should know after 10 years of using.

FYI: iPhone 4s is over 10 years old.

I'm senior myself (over 70). What's your point.

FYI: Backup can only be done by you or someone that you let them do for you.

So it's like asking here "How will I know if my plates are washed".