Paste a PEM certificate — or a whole chain — and see what's inside: subject, issuer, validity, key type, alternative names, extensions, fingerprints. Certificates get pasted into server-side decoders all the time; they are public-ish, but the habit is bad and the metadata is nobody's business. Here, nothing you paste leaves this page.
BEGIN CERTIFICATE blocks, or bare base64 DER. Private keys are recognised and refused; CSRs belong in the CSR decoder.