What is the test for making a motion to suppress an involuntary confession?

Defendant contends his confession to police was involuntary, but he acknowledges that he cannot raise this issue directly on appeal because his trial attorney did not move to suppress his confession. Accordingly, he contends his trial attorney was constitutionally ineffective for not making such a motion. To prevail on this argument, defendant must show that the motion to suppress he contends his trial attorney should have made would have had merit. (See People v. Wharton (1991) 53 Cal.3d 522, 576.) Defendant has not made this showing.