Notice the way “I” reduces from /aɪ/ to /ʌ/ because it’s weak. It’s important not to remove your tongue between the two “t”s. For the first one, stop your voice, but don’t release the /t/. Keep your tongue in place, then release the /t/ straight into the /e/ sound. It’s a bit like just having one long “t” instead of two.
In this phrase, “tell” means “see”, so it means “I can’t see what/who it is.” For example, “Is that Bob over there?” “I can’t tell. I haven’t got my glasses.”