How can I advertise my metal fabrication business in Mexico for exports to the U.S.?

The best way to test the market would be to start with direct mail to lists of business decision-makers in California and neighboring states. Start with a clear target audience in mind--from the type and size of business that could buy from you to any other special qualifying criteria that would make a business a good prospect. 

Then contact a list broker in California and provide this information, as well as the title of the individuals (purchasing director, vice president, president, etc.) who can make a purchase decision. The list broker will search for the best lists for your purpose and will generally be compensated by a commission from the list vendors or a modest fee.

I suggest you send a letter with a brochure that pictures your factory and makes your pitch "real" for the recipient. Also, be sure to have a great website in place. Business prospects will spend a lot of time there to validate their impressions of your company.

Once you have determined this to be a solid marketing course, I suggest you advertise in trade magazines that reach your best prospects. The combination of direct mail, a solid website and trade magazine ads will add up to a well-rounded campaign.