Use a comma before too if you aim to turn the sentence into a double entendre. The change in meaning is subtle and lends itself well to being snide. It's not so subtle this is difficult though. It's pretty straightforward.
It's a bottle. - description of an object
It's a bottle too. - bottles bottles more bottles
It's a bottle, too. - it's a pencil case and it's a bottle
I like you. - thank you so much
I like you too. - mate, we're your fandom
I like you, too. - but I'm also very jealous of you
I like you, too. - but I'm married to your your sister.
I like you too. - we feel the same about one another
The classics (no need for commas - spoken English alert). Requires common sense. Potentially abusive:
It was a pleasure to meet you, too.
It's lovely to hear from you, too.
I love you, too.
I missed you, too.
(Be especially wary. Comma too = as well something else )