You two have been friends for the longest time and obviously, there's some romantic connection that hasn't been addressed yet. You like her and you know that she likes you too. But how to be sure of this?
Both of you are gladly spending the most romantic evening of the year together. You may not have grand plans but sitting at home and watching your favorite shows is also a big deal.
Over the years you have known each other, you'll have created such a comfort level that it's almost natural to have inside jokes.
Or you have probably met her family already and you're pretty cool with them.
If her friends are excited to meet you, then she probably talks about you A LOT. That's a sure sign to know if a girl is interested for the next move.
Shall 'we' go check out this new movie together? 'We' should go grab a bite somewhere? 'We' surely testifies both of you being tight as hell.
Maybe just by picking a new shirt or coming along with you for a haircut, but she definitely gives her opinions on such things.
She might come over at your place so many times that she's bound to leave essential items like her toothbrush at your place.
When you're upset, you want to talk about it with her and vice versa. Both of you are so emotionally connected that you'll depend on each other for support.