The best way for Americans to avoid this is don't work for a US based agency. Every party in a contract is going to interpret tax law to their benefit. A US based agency is probably going to claim you are a subcontractor because it is beneficial for them to do so.they will file a 1099 and you will have to pay self-employment tax. In actuality, most countries, including the US, have a "reality test" that legally determines whether you qualify as an employee or subcontractor. This keeps the local McDonald's from calling all their cooks and cashiers sub contractors. It also applies to us as contract pilots.
Best thing to do is just choose a nonUS based agency if you are American.
I don't Use a tax accountant but it might be a good thing to do if you don't understand US tax law for expats. It gets a bit complicated.