There is no overlap between real Pampas Grass (Cortaderia) and Fountain Grass (Pennisetum). The smallest Pampas Grass is much larger and has much larger flowers than the largest Fountain Grass.

The best way to learn to ID the larger grasses from a drive-by is to study Google Images. A few to look up would be Cortaderia selloana, Miscanthus sinensis, Panicum virgatum, Saccharum ravennae, Arundo donax and Sporobolus wrightii.

Get to know things like their foliage height, flowering height and time of flowering. This will help you to name what you see while driving by.

Albert, if you are really in Sunset zone 2 or 3, I doubt what you are seeing is pampas grass - it is not hardy below USDA zone 7 (dwarf varieties maybe to zone 6), which translates roughly to Sunset 4.

The other varieties Donn lists should be hardy to at least USDA 6, some lower. I would think these species would be more common in your area than the Cortaderia.