Click on contact name in chat header, then click the OMEMO Fingerprints tab to see their device fingerprints.
OMEMO Fingerprints
Read the fingerprint to your contact via another secure channel (phone call, in person). Each fingerprint looks like:A1B2 C3D4 E5F6 7890 ...
A1B2 C3D4 E5F6 7890 ...
If fingerprints match, click Trust next to the device. The lock icon in chat turns green = verified E2E encryption.
Trust