The Easiest Solution:
Follow along when everyone else starts clapping.
This rule works surprisingly well! In any audience, there are usually seasoned aficionados who know the piece well enough to clap at just the right moments. Following their lead generally does the trick.
By Genre:
Concert
In classical concerts, it’s customary not to clap between movements of a single piece—especially in large, serious works like symphonies, concertos, or sonatas. These pieces are designed to flow uninterrupted, creating a cohesive experience for the listener.
Concert has the encore design that at the end, prolonged applause—and the hope for an encore, is welcomed—as long as the principal performer hasn’t left the stage. If they’re still there, keep clapping, and you might get rewarded.
Opera
Opera has its own traditions. Singers typically prefer not to be interrupted by applause. But, of course…
When the audience hears a particularly breathtaking aria or a powerful chorus, applause and cheers are likely to fill the hall. And if a visually stunning scene is revealed on stage, expect applause then too.
Ballet
Ballet is perhaps the most audience-friendly in terms of clapping rules: here, spontaneous applause is encouraged. The energy from the audience feeds back into the performance and is usually crucial for a brilliant night.
Audiences clap when the conductor enters the orchestra pit, to greet the musicians. Applause often follows major characters’ entrances, impressive solos, and especially remarkable moves. I’ve even seen rhythm-matching claps during coda performances—a unique vibe you’ll only see in ballet, or perhaps in the encore of a New Year’s concert’s radetzky march.