What is the test for requiring a certificate of probable cause in a criminal case?

"The purpose for requiring a certificate of probable cause is to discourage and weed out frivolous or vexatious appeals challenging convictions following guilty and nolo contendere pleas. [Citations.] The objective is to promote judicial economy 'by screening out wholly frivolous guilty [and nolo contendere] plea appeals before time and money is spent preparing the record and the briefs for consideration by the reviewing court. [Citations.]'" (People v. Panizzon (1996) 13 Cal.4th 68, 75-76.)