Wall Socket

Cable providers such as Comcast, Charter (Spectrum), etc. are regional carriers and have agreements that dictate what areas they can and cannot service.

If you do not have a cable toning tool or cable tester but you do have a Comcast provisioned cable modem from a previous installation, hook said cable modem to each outlet. If it locks in, it's a live Comcast outlet. If not, either the outlet is not patched in (live) or it could possibly be from a DirecTV, DISH or another provider. 

Here is another thread with more information - https://forums.xfinity.com/t5/X1/How-test-the-end-of-a-coax-cable-run-to-see-if-signal-being/m-p/3186394/highlight/true#M161673

You could plug a cable modem into each outlet to see if any are active.  They may all route back to a common location in/outside the apartment building where Comcast would have to reconnect/connect to make them active on a current account.

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@LASQ wrote:
Newbie question. We just moved in a new apartment which have Comcast before. However, there are 4 wall sockets for the cable how would I know which one is for the Xfinity coaxial cable. Thanks a lot 👍🏽

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You could simply schedule a professional install and they'll take care of any and all required outlet connections for you. (Fees might apply)