您可以运行以下代码：

JavascriptExecutor jsExec = (JavascriptExecutor) driver;
Boolean isScroll = (Boolean) (jsExec.executeScript("return document.documentElement.scrollHeight>document.documentElement.clientHeight;"));

重要的部分是检查浏览器高度的JavaScript代码。
完整代码：

WebDriver driver = new ChromeDriver();
driver.get("https://selenium.dev");
JavascriptExecutor jsExec = (JavascriptExecutor) driver;
Boolean isScroll = (Boolean) (jsExec
        .executeScript("return document.documentElement.scrollHeight>document.documentElement.clientHeight;"));
if (isScroll == true) {
    System.out.print("Scrollbar is exist.");
} else if (isScroll == false) {
    System.out.print("Scrollbar is not exist.");
}
driver.quit();