A Domain Expiring, how to see when

WhoIs, the tool to use.
To see the publicly visible Domain registration details is with WhoIs, a general DNS tool to check domain registry. All Domain registrars run this, though not all show all the relevant information for this exercise. If they just say if it is available or not without showing EPP Status Codes, then try a different one. I primarily use DomainHelp, or if possible the registrar for the client.

So you see a domain you want that is currently registered, but you can find if it is expiring (forgot to renew or abandoning) when you check the dates. There are three dates that should show for a domain as per this example

Updated Date: 2023-11-02T07:27:58Z
Creation Date: 2022-09-19T18:03:11Z
Registry Expiry Date: 2023-09-19T18:03:11Z

So if today is 2023-12-02, this shows that the domain has not been renewed, and there has been no DNS service for it for over two months. It isn't finished being release yet, but it is getting close.
To check exactly where it is in the expiry and release process, look for the EPP Status Codes in the WhoIs listing, such as

/epp#clientTransferProhibited
/epp#redemptionPeriod

The first one is a good one for all actively used Domains, the second one shows that it is in the 30 day time the owner could still renew the domain.
So in the example case, any point on or just after 2023-12-02 it will change to

/epp#pendingDelete

At this point you want to be checking/refreshing the WhoIs looking about as often as you want/need that domain. A few times a day at first, to hourly or more on that 5th day.
When the WhoIs look up changes to Available, Go Register it NOW, do not pass GO, do not do anything else, as if someone else is looking to grab it, they may beat you.