That article was helpful, thanks.  For others benefit I've written the following class that checks to see if a given user has "read all" permission to a given sObject. and if so, reports the profile name or permission set name that granted him the access.
 

public class PermissionChecker {
    // report whether a given user has 'Read all' permission on a particular type of sObject
    public static PermissionReport HasReadAllPermission(String sObjectName, Id userId) {
        PermissionReport r = new PermissionReport();
        List<PermissionSetAssignment> psas =
            [SELECT Assignee.Name, PermissionSet.Id, PermissionSet.isOwnedByProfile, PermissionSet.Profile.Name, PermissionSet.Label
            FROM PermissionSetAssignment WHERE PermissionSetId IN
            (SELECT ParentId FROM ObjectPermissions WHERE SObjectType = :sObjectName AND PermissionsViewAllRecords = true)
            AND Assignee.Id = :userId];
        r.HasPermission = psas.size() > 0;
        if (r.HasPermission) {
            PermissionSetAssignment psa = psas[0];
            r.IsProfile = psa.PermissionSet.isOwnedByProfile;
            r.Label = r.IsProfile ? psa.PermissionSet.Profile.Name : psa.PermissionSet.Label;
        }       
        return r;
    }
    
    public class PermissionReport {
        public boolean HasPermission {get; set;}  // true if have permission via profile or permission set
        public boolean IsProfile {get; set;}  // true if profile, false if permission set
        public String Label {get; set;}  // the name of the profile or permission set granting access
    }
}

Example usage:

PermissionChecker.PermissionReport pReport = PermissionChecker.HasReadAllPermission('Account', UserInfo.getUserId());
if (pReport.HasPermission) {
    if (pReport.IsProfile) {
        System.debug('You have read all permission via profile ' + pReport.Label);
    }
    else {
        System.debug('You have read all permission via permission set ' + pReport.Label);
    }
}
else {
    System.debug('You do not have read all permission');
}