I guess it would depend on what system you are working in. But sampling transects are an often used method to identify plant diversity in an area. Basically write down presence/absence of bryophyte species every Xm as you walk along a line Ym long. Repeat several times for replication. Then you could try and fit your data to a species-area curve (diversity on Y-axis, area surveyed on X-axis). Usually, after a given distance, species diversity reaches an asymptote. If your data are asymptotic, you have probably captured most/all of the species diversity with your sampling method. If not, you probably need to sample further to capture the total diversity of the system.