Send to a small set of seed mailboxes you control across Gmail, Outlook, and others, then read each inbox over IMAP to see where the message actually landed — Inbox, Spam, or Promotions. Provider dashboards like Google Postmaster and Microsoft SNDS confirm reputation. Open-tracking pixels infer delivery unreliably; direct placement checks do not.
There are three honest ways to know where mail lands, in increasing confidence. First, provider dashboards — Google Postmaster Tools and Microsoft SNDS — report domain and IP reputation, spam rates, and authentication results for mail you already send. Second, receiver-side placement testing: send a copy to seed mailboxes you own across the major providers, then read those inboxes directly over IMAP and record Inbox vs Spam vs Promotions per provider. This is the only method that observes actual placement rather than inferring it. Third, complaint and bounce signals from your own sending logs. What does not work: treating open-tracking pixel rates as "inbox rate" — opens are suppressed by image blocking and inflated by security scanners, so they tell you almost nothing about placement. Humerly runs the pre-send audit plus owned-seed placement measurement (sample-scoped) so you see setup failures before you send and accurate placement after.