A free email deliverability tester won't catch everything a full audit would, but it's the fastest way to get a read on where you stand before you send to your whole list, and I run most of these myself on client accounts before we make any changes. Here are the ones worth your time, organized by what they're actually good at rather than just listed out.
This is the one I'd install first if you send any meaningful volume to Gmail addresses, because it's the closest thing to a direct read on how Google actually sees your domain, not an estimate from a third party. It shows your domain and IP reputation, high, medium, low, bad, spam rate, authentication success rate, and delivery errors, all broken down over time. The catch is it only works for domains sending real volume to Gmail, and reputation data shows up as a range rather than an exact number, but for a free tool straight from the source, nothing else gets this close to the truth.
Mail-Tester gives you a quick score out of 10 after you send a test email to an address it generates, checking your authentication setup, spam trigger content, blacklist status, and a few technical details in one pass; it's the fastest sanity check before a send. GlockApps goes further by actually placing test sends across a panel of real inboxes, Gmail, Yahoo, Outlook, and others, and showing you where each one landed, inbox, promotions, or spam, which is closer to what your real subscribers will experience. GlockApps has a limited free tier with paid plans beyond that, while Mail-Tester is free for occasional single tests.
MXToolbox is the tool I reach for when something's actively broken, a domain suddenly bouncing, a suspicion of a blacklist issue, because it checks your domain and IP against dozens of blacklists at once and flags DNS misconfigurations in the same pass. It's not a placement tester in the way GlockApps is, it won't tell you whether you're landing in Promotions versus Primary, but it's the fastest way to rule out, or confirm, a blacklist as the cause of a sudden drop. If it does turn something up, how to remove a domain from blacklist walks through what to do next.
Beyond the general testers, it's worth running a dedicated check on your authentication records specifically, since a lot of placement problems trace back to a misconfigured SPF, DKIM, or DMARC record rather than content or list quality. MXToolbox, dmarcian's free DMARC checker, and the authentication section inside Mail-Tester's report all do this well. If you haven't set these up properly yet, or you're not sure whether your current setup passes alignment, SPF, DKIM, and DMARC setup covers the configuration itself; the checkers here are just for confirming it's actually working the way you think it is.
None of these tools sees the whole picture on its own, they each check a different slice: Postmaster Tools shows Google's real view of your domain over time, Mail-Tester and GlockApps show placement and content-level issues on a single send, and MXToolbox rules blacklists and DNS problems in or out. I'd run a Mail-Tester check before any meaningful send, keep Postmaster Tools open in a tab if you send real Gmail volume, and pull up MXToolbox the moment something looks off. Free tools like these are genuinely useful for a spot check, but they're a snapshot, not a diagnosis, and if the pattern keeps repeating across sends, that's usually the point to get a proper audit rather than keep spot-checking one send at a time.
Google Postmaster Tools is the most accurate for your Gmail-specific reputation because it comes directly from Google, but pair it with GlockApps or Mail-Tester for a broader view across other providers.
Yes, GlockApps and similar seed-list testers show placement by folder including Promotions, while tools like Mail-Tester mostly focus on spam versus inbox rather than the Promotions tab specifically.
If you're sending consistently and need ongoing monitoring rather than one-off checks, most senders eventually move to a paid tier or a managed service, since free tools are built for spot checks, not continuous tracking.
The free Inbox Scorecard at /scorecard runs a lot of this in a single pass, and if it flags something worth digging into, a full audit at /audit is where I go deeper with you.