Decode the IMEI (free, keyless), validate it, and get the seller-claim-vs-record contradiction — plus the one thing most "free IMEI checkers" hide: a clean free result does NOT mean the phone is clean.
Find the IMEI: dial *#06#, or Settings > General > About (iPhone) / Settings > About phone (Android). MacBooks with cellular: About This Mac > System Report > Hardware.
No free or keyless IMEI blacklist check is complete. Public/free checks query only the major shared databases and MISS roughly 30-50% of reports — blacklist status is carrier-specific and country-specific, and a device clean in one country can be blacklisted in another. A "not found" result here therefore means UNKNOWN, NOT clean and NOT safe. The only authoritative check is the full GSMA blacklist (e.g. CheckMEND), which is a paid lookup. Always run it before paying for a used phone.
What you CAN confirm for free: the IMEI is structurally valid (Luhn), the TAC matches the claimed make/model, the device boots to setup with no Apple-ID / Google-account lock, and a different-carrier SIM is accepted. What you CANNOT confirm for free: that it was never reported lost/stolen/blacklisted — that needs the full GSMA check.