Hi Melissa, I am not a native English speaker, but I believe you should use I can tell for situations in which you could easily deduce its content. For instance, if you're looking at the clouds and they look dark and grey you could say "I can tell it's about to rain" or I suppose it's about to rain. It is also common to use it in more sarcastic situations such as seeing a friend of you really mad and saying "I can tell you're very happy today". In general, I would consider it similar to I suppose or I can see that... or I noticed that... Hope it helps!

Native English speaker from the U.S. here. "I can tell" is a way of saying that something is evident or obvious to you. It's a way of talking about something you have perceived or observed. Jackson: I'm sick. [sneeze, cough] Maggie: Yeah, I can tell. Mike: I've been really happy lately. Anna: I can tell. You seem to be feeling a lot better about your job. I can tell this meeting is going to run long. She is really talkative today. I can tell you want to say something. Go ahead and say it.