Go to the Sonos My System page and they are listed there. You have to sign in there.

https://www.sonos.com/myaccount/system/households/devices/

I do not see what would indicate the generation.  There is the device, registration date, serial number, and an indication of modern product but none spell out the generation. Below is an example.
 

Play:5 Dining Room

Registration: 11/20/2017

Serial: 

Status: Online

• Modern product

@ChazW, remove the serial number for privacy reasons.

Playbar, Play:3, Sub are 1st Gen. ; the Play:5 2nd Gen is easily to distinct from the 1st Gen. due to its casing.

ChazW your Play5 MUST be a Gen2 or it would be labled as a Legacy device..

I thought Sonos only recently announced that gen 1 would stop receiving updates and it hadn’t taken effect yet so I would expect the products to still indicate modern.  All my products indicate modern including the play 3s.

Only specific products are effected.

Sonos legacy and modern products

@Smilja thanks for the info and that is really disappointing as I have had the gen 1 you list for only about 3 years.

@Smilja again thanks for the info. I feel like I dug around quite a bit but did not get to that page.

Hello @ChazW. Sorry for the delay, and welcome to the Community. From what I can tell in this thread, your Play:5 is a generation two model, which is why it is labeled as a modern product. 

Also as @Smilja mentioned, if your Play:5 looks like the picture they posted, then it is 100% a generation two speaker, which will not be affected when legacy products cease to be updated in May. If you are still not certain, feel free to send me a private message with the serial number. After that, I will be able to confirm this for you.