Easiest way: ask for a referral to an attorney practicing law in the United States. Or ask a family member or acquainted. The most important part in hiring a Mexican lawyer is trust. If the client-attorney relationship goes sour, the options to make the attorney in Mexico accountable will be hard to get and costly. Prevention is the best medicine.
How will you two communicate? Language harmony
English-Spanish fluency is a must either on you, your attorney in the US, or your attorney in Mexico. Communication will be complicated if fluency in both languages is not attained.
In my opinion, the best option is to find an attorney licensed to practice law in Mexico AND in one of the states of the United States. Not only they will understand the legal and cultural differences between countries, but will have ethical foundations similar to those of the attorneys in the US.
Make direct contact before hiring and transferring money
Before hiring the attorney licensed in Mexico, and before transferring funds, you should make e-mail and telephonic contact. And if the legal matter involves a high sum of money, consider traveling to Mexico to meeting the candidate in person.
Put it in writing
Communicate expectations in advance to MexCounsel. In writing. And have a retainer agreement with a fee schedule. In writing. Best case scenario: fees are agreed upon before starting the case. Worst case scenario: fees are determined along the way, in a "charge at will" basis.
The smell test
Trust your instincts: if you do not feel comfortable with the candidate, go to the next one. I cannot overemphasize how many headaches will be saved if you hire somebody you trust. The time spent finding the right attorney licensed to practice law in Mexico is your best investment.