A small userscript which will mark artists without artist pages when viewing posts. Also marks artists whose urls contain direct images. Those probably aren't correct.

artistchecker.user.js

// ==UserScript==
// @name         e621 artists
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  notify if artist page is missing
// @author       Earlopain
// @match        https://e621.net/posts/*
// @icon         https://www.google.com/s2/favicons?domain=e621.net
// @grant        GM_setValue
// @grant        GM_getValue
// ==/UserScript==

(async function () {
    'use strict';

    const tags = document.querySelectorAll(".artist-tag-list .search-tag");
    const alreadyCheckedOK = GM_getValue("alreadyCheckedOK", []);
    let updateChecked = false;
    for (const tag of tags) {
        const tagSanitized = tag.innerText.replaceAll(" ", "_");
        if (alreadyCheckedOK.includes(tagSanitized)) {
            continue;
        }

        if (await shouldCheck(tagSanitized)) {
            tag.style.color = "red";
        } else {
            alreadyCheckedOK.push(tagSanitized);
            updateChecked = true;
        }
    }

    if (updateChecked) {
        GM_setValue("alreadyCheckedOK", alreadyCheckedOK);
    }

    async function shouldCheck(tag) {
        const ignoreList = ["unknown_artist", "conditional_dnp", "anonymous_artist", "sound_warning", "avoid_posting", "unknown_artist_signature"];
        if (ignoreList.includes(tag)) {
            return false;
        }
        const url = "https://e621.net/artists.json?search[name]=" + tag;
        const request = await fetch(url);
        const json = await request.json();
        if (json.length === 0) {
            return true;
        }
        const artist = json[0];
        if (artist.urls.length === 0) {
            return true;
        }
        for (const entry of artist.urls) {
            const url = entry.url;
            if (url.endsWith(".png") || url.endsWith(".jpg") || url.endsWith(".jpeg")) {
                return true;
            }
        }
        return false;
    }
})();