This question has been answered by cwhissen. See the first response.
They can see the cached output by right-clicking on the resource in the Tree, "Overview", and the Cache Output tab. This will show what's cached, which will include any uncached MODX tags.
Of course, if a resource has just been created or edited, and the cache cleared, there won't be any cached version to view. So viewing the page, then viewing its source, is still the best way available.
<pre> // all template code </pre>
$modx->resource->_output = '<pre>' . $modx->resource->_output . '</pre>'; return '';
,viewsource: function() { window.open("view-source:"+this.config.preview_url); return false; }
btns.push({ text: _('source') ,id: 'modx-abtn-viewsource' ,handler: this.viewsource ,scope: this });