This is a great question, and applies to much more than just bikes. 

For me, I'm lucky enough to be at the point in my life where I can afford US built/made goods if I want to. Maybe not all the time, but if I really want to get the US made version, I can usually swing it.  But I'm under no false premise that the US made, or UK made, or EU made product, is any better or worse than a Taiwan, China, Thailand, etc made product.  Or even the Designed in USA but build in China. The product and product line and the values of the company as a whole dictate the physical qualities of the product, not where it is made.   I've even been to factories in Taichung and met the owners of the factories and builders of the frames, and they are people just like anywhere else.

So how to choose.  I guess I try to buy US when I think the money will have more impact to go back into the pockets of the workers.  But typically before I by US I try to do some research, even call or email and talk with the company before buying to see what the people are like and how things work.  Sometimes asking very blunt questions, which I find people who treat their employees well have no problem answering and are generally more than happy to brag about it.  If the owner cares, employees tend to be cared for.

Is there a way to know 100% of the time, no.  But I try to do my best to put my money where I think it makes the most impact and with people who treat their employees well and care for similar issues to my own.

Think that is about all that can be done.