Somebody knows the if conditions of

[Opserty]

You could just check to see if the variable has a value like...

<if condition="$somevar">
    The value is: $somevar
</if>

What is your current code?

An alternative would be to check the script name like:

<if condition="in_array(THIS_SCRIPT, array('showthread', 'showpost'))">
    The value is: $somevar
</if>