If you know the species or genera of the plant, you can always look at the genomic sequences of related plants for alignments and comparisons. Sufficient sequencing data should be available on UCSC genome browser, Ensemble etc.
The only precise way of identifying the introns (in case the information is not available already) is to amplyfy the respective gene from cDNA. Once you have the full cDNA sequence of the gene, you can align it with the gDNA sequence and thus identify the introns.