Q:

How can users identify which of their iPhone/iPad apps are still only 32-bit (and therefore won't work in future)? 

Many iPhone/iPad users still use apps on their devices which are only 32-bit.  These apps will not work in future if users upgrade to iOS 11, unless of course the apps themselves are updated to run natively in 64-bit. 
This being the case, users may want to identify any important apps they have installed that will become useless if they upgrade to iOS 11 in a few months.  By doing this users can prepare for the upgrade by:

contacting the developer to confirm their future development plans, and
finding alternative apps that will meet their needs

How can users identify which of their currently installed apps are still only 32-bit? 

A:

In iOS 10.3 and above, users can: 

Go to Settings
Swipe up and tap on General
Tap on About
Tap on Applications 

A list will appear showing all of the apps you have installed that 
won’t run in 64-bit mode.  Now you can tap on any of your important apps 
to determine if it is still available on the App Store.  If…

you get a message indicating it is not available, it’s quite likely 
that the app is no longer supported.  You should try contacting 
the developer to check whether it will be updated.  
you’re taken directly to the App Store, then the app is still 
available and you’ll need to contact the developer to confirm 
their plans for 64-bit support.  

This process is the recommended approach for ensuring you 
don’t get caught out when iOS 11 is released later in the year. 