Taxes are calculated during sell price calculation at the unit price level for each product in the cart. The taxed amount is stored in the actual price field using Drupal Commerce's price components system, which tracks the various amounts that got totalled up into a product's price, line item unit price, or order total price.
Since tax data is stored in the price field data along with other price components that result from actions in the price calculation process, you won't find it on its own line item.
This data is available via the API using the commerce_price_component_total() function. You just have to know the name of the price component type you're looking for inside the order total price field's data array.