Prevalence answers a human question
If you ask whether a word is common because you want readers to understand it, prevalence is often closer to the real question than frequency. It estimates recognition, not just occurrence.
A word may be frequent in specialist writing but unfamiliar to many readers. Another word may appear less often but still be known by nearly everyone.
Use search
Search any English word to see the percentage known and rarity label. Indexed word pages add definitions, examples, related words, and similar-familiarity comparisons.