You can test pond water for nitrate and phosphate levels with the correct kit.

Understanding these levels, will help guide you to the correct fertilizer to get a plankton bloom.

In this past thread, Greg Grimes mentions using a 10-52-4 fertilizer and Bill Cody talks about understanding your water to know which variables may affect the result.

https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=205844

Maybe I am overthinking this but I would rather have a wealth of information before I applied fertilizer to ensure I get the proper desired result.