I was wondering, when buying tokens on ICO, is there any 'guarantee' that the token will be listed on an exchange?
there is no guarantee.
In other words - how can one know for sure that an ICO token/coin will be included on an exchange after the ICO is finished?
no investor can know when the token can be listed in exchanges. only developers who know this and they will give an announcement when the token is listed in an exchange.