If you ultimately want to use it in cells, you might want to see if the PKA inhibitor can inhibit CRE-luciferase or any CRE driven reporter in your cell of interest upon activation with agonists.  If you only intend to use the inhibitor in vitro, validating its activity in a standard biochemical assay monitoring kinase catalyzed substrate phosphorylation might be adequate (eg using commercially available kits from Promega , Millipore etc)

Thanks Rishi! Yes, I'm gonna use it in my cells (C3H10T1/2). Initially I was planning to treat my cells with PKA inhibitor and then forskolin. I will do an IP to pull down creb with an anti-creb antibody. Followed by WB using anti-phosphor serine antibody. But heard that anti-phosphor serine antibody is not a good antibody so I need to try another method to do the control...