This plugin will work for custom plugins if they are properly developed, according to Oxwall code standards. You can send me a plugin and I'll test for compatibility.

What can I check in the code to verify that it will work?

You can check how forms are coded in your plugin template files (html files).
To work correctly with this plugin there should be something similar to the following:

{form name='videoAddForm'}
        <table class="ow_table_1 ow_form">
            <tr class="ow_alt2 ow_tr_first">
                <td class="ow_label">{label name='code'}</td>
                <td class="ow_value">{input name='code'} <br /> {error name='code'}</td>
                <td class="ow_desc ow_small">{text key='video+code_desc'}</td>
            </tr>
            <tr class="ow_alt1">
                <td class="ow_label">{label name='title'}</td>
                <td class="ow_value">{input name='title'} <br /> {error name='title'}</td>
                <td class="ow_desc"></td>
            </tr>
            <tr class="ow_alt2">
                <td class="ow_label">{label name='description'}</td>
                <td class="ow_value">{input name='description'} <br /> {error name='description'}</td>
                <td class="ow_desc ow_small">{text key='video+description_desc'}</td>
            </tr>
            <tr class="ow_alt1 ow_tr_last">
                <td class="ow_label">{label name='tags'}</td>
                <td class="ow_value">{input name='tags'}</td>
                <td class="ow_desc ow_small">{text key='video+tags_desc'}</td>
            </tr>
        </table>
        <div class="clearfix ow_stdmargin"><div class="ow_right">{submit name='add' class='ow_ic_add ow_positive'}</div></div>
        {/form}