This is a question that has plagued many young composition students throughout music history. It may seem like a simple matter on the surface level, but once you start combining idiomatic concerns with timbre and dynamic considerations the issue becomes increasing complex.
There are a few easy solutions to this problem:
- Start by writing for instruments you know. It’s easier to make sure something is possible on an instrument if you’re writing for something you know how to play. It’s even better if you sit down and play through what you’ve written!
- Befriend instrumentalists of various kinds and ask for them to play through your music. If they can add notes detailing any difficult passages or make suggestions that’s even better!
- Practice! Find easy piano music and arrange it for various ensembles. Bring the music to performers and make notes on their feedback. Then edit your arrangement based on their feedback and repeat the process. Every time you make a change you’ll learn something new!