Aren’t you asked which is which when you set them up? Un-pair them and then re-pair them. 

Or toggle the status lights?

Disregard this suggestion- it works on the pair, not individual speaker. See alternative suggestion below. 

Thanks for the replies.  As noted, new to Sonos.  Added them separately as got the second one llater so not asked when added it in.  Will take a look at the status lights.  Was trying to avoid unpairing and adding again but may have to resort to that.  Thanks again!

No problem. There’s a great bunch of help-givers here, so just ask if you need help. 
 

I just realised status light affects the pair! Instead, try toggling a voice service on/off. To do that:

Take a look at Settings (icon far right at the bottom of the app screen) / System / name-of-the-room, then select one of the speakers and scroll down to Voice. 

You can download test tones for your Music Library or find some on many streaming services.

Thanks to all!  Toggled the services for Amazon Music and learned which was which.  Then simply physically reversed the speakers and voila!  Left is left and right is right!  Again, thanks to all for the replies.