As with any financial transaction regarding a contract, you need to protect yourself. Research is your best friend in these cases. Here are some things you can do to find out whether or not you are dealing with a reputable company;
Ask them how long they have been in business.
Chances are; the longer they've Credit card processing ISO programs been in business, the more reputable they are. You cannot stay in business for too long if you have a history of making bad deals.
Look at them through the Better Business Bureau
The Better Business Bureau is a valuable resource. The BBB has been protecting consumers and small business for decades. They should be able to provide you with an objective perspective.
Please note that just because a business does not show up in their database; it doesn't mean they are disreputable. It may just mean they are a relatively new business and they haven't come under the BBB radar yet.