just compare with your print or film to determine density visually. but not good enough.
Is a important tool in the darkroom, where you establish your own standard with exposure/processing and control.
You may DIY. It is better solution.
Use a piece of white / grey / black board, make std and +5/-7 stop exposure process and compare, cut and combine difference exposure film into one strip. Make contact / enlarge print together with the strip, if you have a darkroom exposure meter or densitometer , measure and record every step. This is where you start to have your darkroom standard.
The #5 print exchange is TOMORORW Sat 9 Sep 1030am at the small food court (ground floor) Peninsula Shopping Centre. Come and have a cup of kopi, share your prints, works, books and gear.