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!