I've been running into this trailing の particle quite often and can't seem to understand what exactly it means. Is it replacing か, or does it mean something else?
"Instead of 「か」, real questions in casual speech are usually asked with the explanatory の particle or nothing at all except for a rise in intonation"
To add to what 無色受想行識 said, の is regularly used in conjunction with interrogative words, such as in どこに行くの？or 何を食べるの？ Often when no interrogative word is used, rising intonation will mark that the sentence is a question, such as in 明日、パーティに行く？
Additionally, の can be used to indicate surprise or disbelief. For example,
In this instance, B probably thought A wasn't going to the party, and is surprised to learn that he is.