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.
To put it as directly as possible, based on my experiences and learning, the best way to describe the article の appearing at the end of questions, after verbs, is to convey a stronger sense of curiosity than simply asking the question without it. If you're familiar with the んですか？[noun]なんですか？ construction, it plays a similar role.
You might think of it as:
お手洗いはどこありますか？ ＝ Where's the bathroom?
お手洗いはどこあるの？あるんですか？= Where's the bathroom?(I really need to go!)
何をしている？= What are you doing?
何をしているの？= What in the world are you doing?/What are you doing?(Tell me! I wanna know!)
(Disclaimer: This isn't a super natural phrasing for this question. It was bent to fit my example. お手洗いはどこですか？ is much more common.)