Plugin acces a template variable?

[PhilMcKrackon]

Is it possible for a plugin to use a template variable? I'm very new at this and am trying to get a bit of code to work.

$show['thumb_exists'] = file_exists('/home/mesing/public_html/links/link_images/14_t.jpg');

I use a conditional evaluation in a template that works as needed. If the file exists the condition evaluates as true and when I remove the file it evaluates as false. Is there a way to get the thread id substituted instead of the hard coded number (14)? It's probably simple but has been escaping me.

Also, besides me removing the file to check the evaluation is there a better way to check? I looked all over the CP and couldn't find anywhere to check variable evals.

Thanks,