

Open your hand as far as you can comfortably stretch and fret the lowest string with your thumb.

Use the primary tuning you put this instrument in when performing or recording.

But the precise and accurate way to measure relief is to use the strings as a straightedge. I often see people sighting down the neck from all angles and making some very ballpark assessments about neck relief. We start with checking the "relief" or amount of bend in the neck.
