I find this article, Everything You Don't Know About Tipping, by Tim Urban of Wait but Why, to be the best I've found.
Even those of us who are native US citizens like him (or myself) often find ourselves in "ambiguous tipping situations", where we are not quite sure what the right amount to tip is, and want to avoid blunders such as:
The Inadvertent Undertip, the Inadvertent Overtip, and the “Shit Am I Supposed To Tip Or Not?” Horror Moment.
In Tim's words,
Tipping is not about generosity. Tipping isn’t about gratitude for good service. And tipping certainly isn’t about doing what’s right and fair for your fellow man.
Tipping is about making sure you don’t mess up what you’re supposed to do.
(And though I like to be generous and fair, there's certainly much wisdom in that.)
He interviewed 123 people in New York City working in jobs that involved tipping, and then supplemented with information from "a bunch of readers" and research, including the website of Wm. Michael Lynn, recognized tipping expert and Burton M. Sack Professor in Food & Beverage Management at Cornell University's School of Hotel Administration.
This handy table breaks it down by tipping situation, percentage of customers who typically tip, how much low, average, and high tippers give, what the providers typically think of people who don't tip in that situation (fine, cheap, the worst), what percentage of their salary is typically provided by tips, and other helpful notes.
So basically, decide whether you want to be a low, average, or high tipper, and tip accordingly.
On his page is also included a handy table to help estimate the impact on one's budget of being a low, average, and high tipper, depending on if you're a low, mid, or big spender when it comes to buying tipped services, and other handy notes about factors that should influence the decision, and how it looks from the service provider's perspective.
And, a fascinating "tipping spectrum" of where people in different groups tend to fall on the tipping spectrum, based on a survey of 1000+ waiters (Sample: Best: All-Male Dining Parties, followed closely by Gay Men; Average: All-Female Dining Parties, Young Adults; worst: Teenagers, Foreigners, Coupon Users), and other interesting things about tips.
The TripAdvisor page referenced in @DJClayworth's answer is also an excellent reference, and includes a few situations not included in Tim Urban's table and article. But Tim's table is just about the most convenient and informative guide I've found for most common situations in a format that provides more info on the server's perspective.
Notes I'll add from personal experience:
As a former sometime taxi driver (in Albuquerque, NM, not NYC), it seemed to me that people were being cheap if they tipped on a straight percentage when the percentage would result in a tip under $1, or were stingy about rounding up to the closest whole dollar (esp. when paying cash). (But, I'd never hold it against people who didn't tip; it did impact my bottom line, but I prefer to think the best of people and not to attribute to malice what can attributed to ignorance, poverty, lack of consciousness, etc.)
- As a consequence, I personally feel cheap tipping less than $1 almost
anywhere (except maybe a coffeeshop or similar place where ticket
prices are low and tips are not expected). YMMV: Your Mileage May
Vary. Albuquerque is not NYC, but is certainly fairly representative
of many mid-size non-"Cab Towns" you may find across America.
It's always OK to add more to an automatically included gratuity, if you feel it's warranted, or you want to be a "good" tipper. Only paying the included gratuity puts you in "average" range.
"Doubling the tax" is highly unreliable, as it varies from state to state and city to city, and some localities have none at all.
I'd agree that most servers prefer cash -- and it's not all that unusual to mix payment types (leave cash at table, pay ticket with card, etc.). If you feel so inclined, there's nothing wrong with it, and for better or worse, servers prefer it because they're likely to be able to take it home the same day, and taxes and card fees are not taken out, so they can take home more, leaving up to their own conscience and discretion how much of their tips they report for tax purposes.
- But, if you're paying by card and don't have cash, no big deal, just
leave it on the card.
15% is not "a bit much for California", at least for a restaurant. It's solidly in "low tipper" range anywhere I've been in the USA.
It's true that it can be considered insulting, or at the least very awkward, to give a tip in a "non-tipping situation". Eeesh. :/ But, that's up to your own conscience and knowledge of the situation.