If you have control over your system, then you might use a Discourse web hook to notify your system that the user is activated. But I’m pretty sure that the easy solution is You can check the active field in the user record. I’m pretty sure that it doesn’t get set until they’ve validated their email address.
http://localhost:3000/admin/users/1/jay.json will give you a record like:
{
id: 1,
username: "jay",
name: "Jay User",
avatar_template: "/user_avatar/localhost/jay/{size}/3_2.png",
email: "jay@nowehre.invalid",
secondary_emails: [ ],
active: true,
admin: true,
moderator: false,
last_seen_at: "2020-11-04T20:13:04.240Z",
last_emailed_at: null,
created_at: "2020-10-05T21:14:52.570Z",
last_seen_age: 27.86773951,
last_emailed_age: null,
created_at_age: 2588319.53785305,
trust_level: 1,
.....
},