do not take this code as-is because I just copy-pasted it from an old test and do not really know anymore if it works correctly. It may help though...

jenkins-detect-cron

    def userIdCause = currentBuild.rawBuild.getCause(hudson.model.Cause$UserIdCause)
    boolean buildTriggeredManually = (userIdCause != null);
    //boolean buildIsNightly = userIdCause.toString().toLowerCase().contains("trigger") || currentBuild.rawBuild.getCauses().get(0).toString().contains("BranchIndexingCause")
    def buildIsNightly = false
    currentBuild.upstreamBuilds?.each { b ->
     echo "Upstream build: ${b.getFullDisplayName()}"
     buildIsNightly = true
    }
    //def specificCause = currentBuild.rawBuild.getCause(hudson.model.Cause$UserIdCause) // must be set to null once used because it is not serializable
    echo "userIdCause = ${userIdCause}, buildTriggeredManually = ${buildTriggeredManually}, buildIsNightly = ${buildIsNightly}"  // #debug