How to use toString method of org.openqa.selenium.remote.SessionId class

There is no way to do this in WebDriver (as of v2.21.0). You can try replacing the usual findElement(By) method with an adjusted one that uses JavaScript to highlight the found element:

// Draws a red border around the found element. Does not set it back anyhow.
public WebElement findElement(By by) {
    WebElement elem = driver.findElement(by);
    // draw a border around the found element
    if (driver instanceof JavascriptExecutor) {
        ((JavascriptExecutor)driver).executeScript("arguments[0].style.border='3px solid red'", elem);
    }
    return elem;
}

Now that you got the idea, there's an improved version that restores the original border of the last element when a new one is found and highlighted:

// assuming JS is enabled
private JavascriptExecutor js = (JavascriptExecutor)driver;
private WebElement lastElem = null;
private String lastBorder = null;

private static final String SCRIPT_GET_ELEMENT_BORDER;
private static final String SCRIPT_UNHIGHLIGHT_ELEMENT;

void highlightElement(WebElement elem) {
    unhighlightLast();

    // remember the new element
    lastElem = elem;
    lastBorder = (String)(js.executeScript(SCRIPT_GET_ELEMENT_BORDER, elem));
}

void unhighlightLast() {
    if (lastElem != null) {
        try {
            // if there already is a highlighted element, unhighlight it
            js.executeScript(SCRIPT_UNHIGHLIGHT_ELEMENT, lastElem, lastBorder);
        } catch (StaleElementReferenceException ignored) {
            // the page got reloaded, the element isn't there
        } finally {
            // element either restored or wasn't valid, nullify in both cases
            lastElem = null;
        }
    }
}

And the scripts! I load them from a file using FileUtils.readFileToString().

SCRIPT_GET_ELEMENT_BORDER (IE friendly version taken from this site), it would be way shorter if it used highlighting via changing the background color, say, only the bottom border. But this is the nicest one :).

/*
 * Returns all border properties of the specified element as String,
 * in order of "width style color" delimited by ';' (semicolon) in the form of:
 * 
 * "2px inset #000000;2px inset #000000;2px inset #000000;2px inset #000000"
 * "medium none #ccc;medium none #ccc;1px solid #e5e5e5;medium none #ccc"
 * etc.
 */
var elem = arguments[0]; 
if (elem.currentStyle) {
    // Branch for IE 6,7,8. No idea how this works on IE9, but the script
    // should take care of it.
    var style = elem.currentStyle;
    var border = style['borderTopWidth']
            + ' ' + style['borderTopStyle']
            + ' ' + style['borderTopColor']
            + ';' + style['borderRightWidth']
            + ' ' + style['borderRightStyle']
            + ' ' + style['borderRightColor']
            + ';' + style['borderBottomWidth']
            + ' ' + style['borderBottomStyle']
            + ' ' + style['borderBottomColor']
            + ';' + style['borderLeftWidth']
            + ' ' + style['borderLeftStyle']
            + ' ' + style['borderLeftColor'];
} else if (window.getComputedStyle) {
    // Branch for FF, Chrome, Opera
    var style = document.defaultView.getComputedStyle(elem);
    var border = style.getPropertyValue('border-top-width')
            + ' ' + style.getPropertyValue('border-top-style')
            + ' ' + style.getPropertyValue('border-top-color')
            + ';' + style.getPropertyValue('border-right-width')
            + ' ' + style.getPropertyValue('border-right-style')
            + ' ' + style.getPropertyValue('border-right-color')
            + ';' + style.getPropertyValue('border-bottom-width')
            + ' ' + style.getPropertyValue('border-bottom-style')
            + ' ' + style.getPropertyValue('border-bottom-color')
            + ';' + style.getPropertyValue('border-left-width')
            + ' ' + style.getPropertyValue('border-left-style')
            + ' ' + style.getPropertyValue('border-left-color');
}
// highlight the element
elem.style.border = '2px solid red';
return border;

SCRIPT_UNHIGHLIGHT_ELEMENT

var elem = arguments[0];
var borders = arguments[1].split(';');
elem.style.borderTop = borders[0];
elem.style.borderRight = borders[1];
elem.style.borderBottom = borders[2];
elem.style.borderLeft = borders[3];

Any questions, notes, requests and improvements are welcome!