The free X (Twitter) username checker
X, still called Twitter by most people, has been handing out @handles since 2006, so short and obvious names went long ago. This Twitter username checker (or X username checker; they're the same thing) tells you in seconds whether a handle is taken, without logging in. It checks the name against X's rules as you type, then asks X whether a public account already uses it.
X handles are up to 15 characters, using letters, numbers and underscores. That's tighter than Instagram's 30 or TikTok's 24, so a name that works elsewhere may be too long for X. If you want one handle everywhere, check the X version first: it's usually the hardest to fit.
A handle that looks free might still be held by a suspended, protected or recently deactivated account, which X doesn't show publicly. If X won't accept a name this tool shows as free, that's the usual reason. Every search also checks Instagram, TikTok, YouTube, Bluesky and the matching .com, so you can see your whole set at once.