How can I check the exon composition of mRNA isoforms in the NCBI database?

I am doing a CRISPR Cas9 with guide RNAs designed to target the second exon found in the "majority of the isoforms" (according to the gRNA design tool). I want to double-check the isoforms and see which ones contain exon number 2. I know I can download the FASTA of the exons but it doesn't tell me the number of the exons and I have to download and compare all isoforms separately. I was wondering if there is an easier way of checking the mRNA isoform "composition".