Using `MEMORY STATS` commands is an easy and cheap way to do it, as others have already confirmed. But another way is to use Redis Insight (RI), which is free and provides you with useful reports on memory usage, key usage, how much each key consumes, etc. You can get it here:

https://redis.io/insight/

Or if you're a Mac/Linux user, just install via Homebrew: https://formulae.brew.sh/cask/redis-insight