what when displaying a variable with kint crashes?
Usually D8 $variables contain way too much data, and browser struggle to render it for you.
I strongly suggest to use xdebug or similar debugger, all modern IDEs support it. It makes developer's life much easier.
However, there is a workaround without having xdebug configured:

function my_theme_preprocess_field(&amp;$variables, $hook) {
    var_dump(array_keys($variables));
    // or better:
    kint(array_keys($variables));
    // or:
    dpm(array_keys($variables));
}


This will return you top-level keys, then you can drill down to the next level: 

dpm(array_keys($variables['some_key']));


For objects use type casting:

dpm((array) array_keys($object));


Set Kint as Variables Dumper on /admin/config/development/devel (with Devel Kint module enabled) and you'll get nicely structured output.

In Twig templates you can use something like this:

{% for key, value in content %}
    {{ key }}
{% endfor %}


UPD: Valuable comment from Kevin for TWIG:

{{ kint(content|keys) }}