In Washington State, you must be licensed to use the word engineer in your title.
Internally, it is a grey area, because there are plenty of people doing engineering that are not licensed. However, if a business card or documents show the word Engineer, they must be licensed.
Designer is one way, engineering, is another way around this. ie Mechanical Designer, Mechanical Engineering....
There are some fields where there is no professional license, such as Quality Engineer, then they can be called an engineer, but that is stretching it with the Board of Licensing.
Only recently, was there a PG title, Professional Geologist, but people had been working for years as Geotechnical Engineers without issue.