Here's a good paper for taking field augur samples and estimating root mass:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5014753/pdf/main.pdf

In glasshouse trials in popts the procedure is to lanoriously wash out the roots.  First soak the pot in water then wash over a sieve to retain as much fine root as possible

where is the problem, especially when doing it in a glasshouse trial? Like Paul says: wash, wash, wash...  :-)