It is unethical and inaccurate for an attorney to guarantee any results.
If someone tells you they have a 100% success rate, make them put it in writing
Many attorneys will say they have never lost a case but few will put that in writing or in your contract.
Hire an immigration lawyer who only practices immigration law
Immigration Law is such a complex and ever-changing area of law that it would be difficult to be a top notch immigration lawyer while practicing other areas of law.
Call their Bar Association and check to see if there is any record of misconduct or client complaints
Note that immigration law is federal and therefore an immigration lawyer may be barred by any state, not just where the lawyer is practicing. Ask the lawyer where he or she is barred and then CALL the bar association to ask about client complaints or any incidents of misconduct. This is also a good way to ensure your lawyer is also actually an active, practicing lawyer in good standing.