Several characters not present on the keyboard can be typed through a key combination. Such combination can be easy found by using the keyboard viewer.
Display Input Menu
In System Preferences | International take the "Keyboard Viewer" checkbox active, in order to show the viewer entry in the input menu.
Activate the viewer
In the Input menu choose "Show Keyboard Viewer", this will activate the Keyboard Viewer palette.
Using the Keyboard Viewer: find out combinations
When pressing modifier keys, like:
one modifier or more than one at the same time, the Keyboard Viewer will change the characters on keys accordingly. Key combinations can be easily detected by looking at keyboard map while pressing the different modifiers.
Using the Keyboard Viewer: find out sequences
When a modifier is pressed some keys can be highlighted: this means that the by tyiping such combination you are activating a sequence.
An example is ⌥-"8" ("´") followed by "a": "⌥"-"´" + "a" → "á"