To check if the current page is a node view page, even if it has a path alias, use arg():

if (arg(0) == 'node' && is_numeric(arg(1)))

You can then load the node with

$node = node_load(arg(1))

node_load() caches the result of such calls in memory, so you can call that from different places and all of the calls together will only grab the node from the database one time.

Using arg() is much better than calling explode('/', drupal_get_normal_path($_GET['q'])) which needs to run a database query.