How do I identify Peyronie's disease scar tissue or plaque? And what is the difference between the 'scar tissue' and 'plaque'?

Same, but ... Plaque is a specific descriptive term to denote the shape of the scar of Peyronie's disease, and scar tissue is a general term to describe the common microscopic picture of healing/healed tissue after injury by various reasons, resulting in disruption of local circulation and cell arrangement. The plaque may widely vary in size & shape so some plaques are barely palpable, and others, obvious.