You can't get this data from the sequencing but if prior to sequencing you (or the one that prepared the libraries) used bioanalyzer or other tool to measure the length of the DNA fragments, you can use this data to compute the length of the inserts.

After you map the reads to the genome you can get the length of each fragment from column 10 (TLEN) of the SAM file, just compute the average of the positive values (>0) in this column.

A little more lengthy of an option: Post-trimming adaptors/barcodes you can align the reads to a reference, or your own de novo assembly using bwa or bowtie. You can then use picard tools (CollectInsertSizeMeterics), and you will get a nice histogram of the insert sizes in your library.