A good rule of thumb is to look up the range of the instrument for which you're trying to write (of course, be sure to determine whether you're looking at concert range, or the range that is transposed to the instrument's key), and then write your music within that range, avoiding notes that are on the high or low end of that range. Notes on the high or low end of ranges tend to be difficult to play because they require more air, or physical finesse, and are harder to keep in tune.
Ultimately, even while keeping these things in mind, you have to know for whom you're writing. You will write differently for seasoned musicians at Fox's scoring stage than you will for a local student ensemble. Something else to keep in mind is that some of the best sounding music isn't overly difficult to play, so writing something that sounds "easy" doesn't necessarily mean the audience won't enjoy it, or that the piece of music is bad.