if you have your mRNA (cDNA) sequence, easily find the introns by performing BLAST of your cDNA against DNA, and non-homologous areas are your introns. Also you can perform BLAST of your primers against cDNA sequence to see if they are 100% homologous or not. If not, they probably contain intron regions.
But if you don't know your mRNA sequence, use following link and you will find a solution: