How do you debug preview env vars that work locally?

A deploy debugging question about build-time and runtime environment variable scope.

Preview deploys often fail with a missing config value even though local works. The useful split seems to be build-time visibility, runtime visibility, project scope, environment scope, and whether a client-exposed variable needs a public prefix. I avoid printing secrets and prefer a redacted “present or missing” check. When a variable works locally but fails in preview, what do you check first?