Is there away to know if a IP address is a Zscaler address?

I need to know if someone is on-site at organization or off-site. I am trying to figure out if I can tell this by the IP address or if there is another way for me to know this. User story: Library resources are authenticated by publishers using IP auth. We want to use a third party dedicated proxies because publishers can’t use Zscaler IPs. We need the third party to know if a user is accessing from on-site or off-site for reporting purposes. We want to limit the number of off-site users and allow for an unlimited number of on-site users.

The IPs of ZScaler can be found here: Config | Zscaler
replace zscalertwo.net with the could you are using on your tenant

But this only tells you if the access to the library ressource happened using ZScaler. Whether that request is coming from an on-prem user or one of your remote users is a different story; depends on your exact configs (network routing, GRE/IPSec tunnel setups, pac files, etc.)