Raycast a specific object

I want to raycast a specific object, but i don't know exactly how, I use this code for the raycast:

if (Physics.Raycast(transform.position, 
    // doSomething

if it's possible i like to use a tag (of the object to see)

See Collider.Raycast.

It will allow you to test a known collider (which can be obtained from any object that has a collider, like target.collider).

Otherwise you can use Layer Masks with Physics.Raycast (see also How do I use layermasks?).