What is the test for probable cause for arrest when investigating drug dealing?

At the very least, the suspicion thus focussed will justify additional investigation. If it is coupled with independent evidence of criminal involvement, it may amount to probable cause to arrest: 'Although previous arrests of a suspect in connection with illicit drug transactions will certainly not suffice to constitute probable cause for search or arrest, and while, indeed, arrests without convictions may be of little probative value, still a suspect's reputation as being involved in illicit drug traffic based on prior arrests may be considered.' (People v. Buchanan (1972) 26 Cal.App.3d 274, 292, 103 Cal.Rptr. 66, 79, and cases cited.) When the investigating officer knows of such a pattern, 'that knowledge can be used, in connection with other information, to support a finding of probable cause for arrest.' (People v. Martin (1973) 9 Cal.3d 687, 692, fn. 5, 108 Cal.Rptr. 809, 812, 511 P.2d 1161, 1164.)