Conditional for 'Is In Template Group'?

[alegend]

Hi,

I am trying to display a widget only when the visitor is within a standard forum page (eg. showthread, forumdisplay, etc.), but not on my custom pages (eg. xx.com/custom-page.php).

All our custom templates are in the 'Custom Templates' group. Is there a way to detect within a template if THIS_SCRIPT is in a template group? Alternatively is there a conditional to check if the template is a custom template?

Thanks in advance,
R.