You’d indeed have to do a test either on the already-deployed functions, or on a local copy you setup using netlify-lambda serve, @nonissue . The functions aren’t available until AFTER deploy for execution via our CDN.
You may be able to pick up some other associated advice from this article such as workflows for testing deploy previews before merging to production, or using multiple sites (1 for test, publishing later to prod): [Support Guide] Testing your Netlify builds