just asking

[aewanko]

when is cross-handing nessecary? I've seen some recordings that need to cross your hands. is it told in the sheet? just wondering

[persona]

Most of the times the right hand is supposed to play the notes written on the set of staves above, and the left hand, the notes written below. In those cases, if you come across some notes written on the set of staves below and they are higher in pitch than the ones being played by the right hand (which are the notes written above), then you'll know that you're supposed to cross your hands. The same happens vice-versa. However, this is not always the case. Sometimes a hand is supposed to play notes written on the set of staves that doesn't belong to it (this happens very often when both hands play a single voice). Some editions that include fingering indications, write the numbers below the note when the left hand is supposed to play them and above the note when the right hand is. If the notes are sixteenth, then it's very common to see a separation when you're supposed to switch your hands, instead of the conventional way of gathering them in groups of four.
But there is no sure way to know by yourself which hand plays what. Most of the times, the best thing to do is ask a teacher for the particular cases you need. Or if you don't have one, asking here might help sometimes (that is, if you are aware of the amount of right answers you can expect to get, and how many you must not consider). And a third way (the one I use the most), look for that piece on youtube played by a professional and see how they play it. That's the best way to learn most things, the good old "watching and learning".

[dan101]

As already mentioned, when notes in the right hand notated are lower than left hand notes that are being played, the hands cross. Similarly, if the left hand notes are notated as higher than the right hand notes that are being played, then the hands also cross. Whether the crossing hand travels under or over the other hand is usually a matter of personal comfort, although I find that I usually cross over. Consult your teacher.