Should dropdown menus be indicated by arrows so users can distinguish between dropdowns and non-dropdowns?
Does this depend whether we're talking headers or bodies, primary or secondary navbars?
Do too many arrows cause visual clutter?

Should dropdown menus be indicated by arrows so users can distinguish between dropdowns and non-dropdowns?
Does this depend whether we're talking headers or bodies, primary or secondary navbars?
Do too many arrows cause visual clutter?

One of the standards I see right now is that an onClick menu often is what's underneath an arrow icon. Often, hovers are done without iconography, because a user can often discover "accidentally." I think adding iconography should probably be used for when a user must take true action and not unintentional action.
As a small aside, I'm not always a huge proponent of onClick menus, but here's a great article on why hover menus have issues (I also really like the term "hover tunnels") http://uxmovement.com/navigation/why-hover-menus-do-users-more-harm-than-good/
Your UI should give a clue for elements that takes a user action so that users can guess how to use it. There should be visual signifiers - this is known as affordance.
So buttons should look like buttons, menus like menus and there should be some sort of clue that a drop down is going to reveal more options when clicked.
This dosn't mean you have to clutter your design with heavy handed design elements - users will pick up on the most subtle of clues.
Your example is essentially a utility nav where dropdowns are not the norm. Also just like here on StackExchange, in the Google example there's several (9) regular links and then one has a dropdown. In those two scenarios the dropdown menu is the outlier so it's critical that you communicate to the user that there's more information under those items.
If it is a site's big old primary navigation then it's much more common for there to be dropdowns and also the user will quickly learn once they mouseover one item that has dropdowns that the others likely do too.
If you compere the gains vs cost, using arrows wirh drop down is the winner by a mile. Visual clutter can be avoided by making arrows 30-50% less visible than the text on drop-downs.