When composing parts for different instruments, how do I find out if the part is playable? [closed]

There's not an easy answer other than what you're asking requires a level of experience and exposure to the instruments in question. As a non-multi-instrumentalist, the quickest logical solution for me is to research possible and comfortable ranges for each instrument and either maintain a cheat sheet, or a list of examples for common instrumentation/tone colors.

The only thing that is a hard and fast rule is if you write a part for an instrument, make sure it is in the playable range of the instrument. Please don't write a low (below the staff) A for a concert flute, for example.

Sensible composition will take the best from experience, and utilize instruments effectively to draw out the correct colors from each respective instrument. For example, you might not want to use a percussive instrument for a sound that is meant to be sustained, like a whole note on a string instrument (unless the idea is for the note to ring out and echo over silence or over other sounds). A lot of composition is using that sense. However, depending on your own ideas, it takes learning the "rules" (more correctly, learning the conventions) and then maybe learning how to break those "rules"/conventions to get the sound you want. In the end, it's your piece! What experience you put behind it will determine if it's a good one.