Poti folosi un conditional tag in tema: is_category().
Citez de pe wordpress.com:

is_category()
When any Category archive page is being displayed.
is_category('9')
When the archive page for Category 9 is being displayed.
is_category('Stinky Cheeses')
When the archive page for the Category with Name "Stinky Cheeses" is being displayed.
is_category('blue-cheese')
When the archive page for the Category with Category Slug "blue-cheese" is being displayed.
is_category(array(9,'blue-cheese','Stinky Cheeses'))
Returns true when the category of posts being displayed is either term_ID 9, or slug "blue-cheese", or name "Stinky Cheeses". Note: the array ability was added at Version 2.5.
in_category('5')
Returns true if the current post is in the specified category id. read more

Note: Be sure to check your spelling when testing, "is" and "in" are a big difference.
In asa fel poti specifica ce sa afiseze daca intri in categoria X sau in categoria Y...

Mai multe informatii: http://codex.wordpress.org/Conditional_Tags