I am new to Western blot, struggling to detect phosphorylated proteins. Please help me out.

What is the best protocol to detect the phosphorylated proteins ? I use phosphatase inhibitor in lysis buffer, I load 20 ug protein. I had to expose the membrane for 15 min during detection to see faint signal, but the membrane gets dark, can I use such long exposure ? Is the data credible ? Please help me, I am struggling. Thanks a lot.