I used the undocumented sp_MSforeachdb in EXAMPLE:1 to Query all Databases at once in one line code, or you can use a cursor for the same as shown in EXAMPLE:2
There may be other effieient methods and maybe some good use of DMV to same approach, but this is what i use as a Quick Reference, when needed.
I have also included the REVOKE Statement for refrerence, incase you want to remove the Guest access to your Database.
EXAMPLE:3 shows the diff methods to the REVOKE approach.
Just Twisting the Cursor statements a bit, we are able to define a all at onec or one row at a time REVOKE method.
WARNING!!! : TEST this on your TEST- SYSTEM First and use this SCRIPT at your own RISK only.
Study your environment before revoking access to GUEST User.