Depositors provide aliquots of their pooled libraries to Addgene and we do not perform any additional amplification steps, so we are confident that the DNA you receive from us is what was described by the depositor. The depositors usually provide information on the pooled library page or in the associated publication describing how they have ensured full coverage.
For example, according to the associated publication, the Sabatini lab ensured full coverage as follows: "To preserve the diversity of the library, at least 20-fold coverage of each pool was recovered in each transformation and grown in liquid culture for 16-18 hours. Similarly, the Zhang lab ensured full coverage as follows: "To ensure no loss of representation, 36 parallel transformations were performed using the same ligation reaction and plated onto 245 mm x 245 mm plates with carbenicillin selection (50 ug/ml), which yielded 166 X library coverage".
Addgene did perform some limited quality control sequencing by transforming one aliquot from each library into E. coli and randomly sampling the resultant colonies to check for proper diversity in the gRNAs present; however, we did not verify the presence of every individual plasmid.