Start from several independent sources.
No single source is the truth on its own. The gain is in combining them: the annual salary guides from recruitment agencies (Michael Page, Robert Walters, Hays), LinkedIn Salary for your own region, tech-specific benchmarks such as Levels.fyi, and open aggregators like Glassdoor or Payscale for a broader picture.
Don't look at the average alone, but at the range — and mind the reference date and the local market. A guide from early last year already lags behind in a tight market, and a figure from one country rarely transfers one-to-one to another.