How to figure out which level I have?

The easiest way I've found is to google the Edmunds reviews of that year Prius and they tell what the added options are for each level.

 

No SKS or backup cam its a base. If you have those its package 2.
if you have leather and fog lights its package 3

 

Smart Key System. If you have to put the alarm fob in a slot to start the car that's a base car. Thats the best part of the car.

 

As mentioned, it's easiest to go by the model year and how it's equipped. The equipment in each option package changed almost every model year. I look in the brochures here: Toyota Prius Touchup Paint Codes, Image Galleries, Brochure and TV Commercial Archives but the resource you posted has it all in one place.

For 2008, you also have to determine if it's a touring or non-touring. They each have their own range of option packages. The easiest way to tell them apart is the wheels (Touring: 16", 7-spoke...Non-Touring: 15", 6-spoke).

Some notes for 2008:

Non-touring:

No base for 2008
Package 1: Basic, no audio system
2: Adds 6-speaker AM/FM/CD, Smart Key System (SKS), reverse camera, Vehicle Stability Control (VSC)
3: Adds Homelink mirror (with garage door remote) and JBL sound.
4: Adds foglights in lower valence and High-Intensity Discharge (HID) handlamps
5: Adds DVD navigation
6: Adds leather surfaces

Touring:

(All Touring variants have foglamps and HID headlamps)

2: Everything but Homelink, JBL, Nav and Leather
3: Adds JBL and Homelink
5: Adds Nav
6: Adds Leather