One-man shows can bring in the elements of temperament, and yours support level will depend on how busy the individual is. With a company, there are a group of people involved who are unlikely to all vanish in a day. With individuals, their personal problems can affect the completion of your project.
Case studies, references, and testimonials
Always ask for one or two case studies and be sure to verify them. Ask for a few references and testimonials. Call up references and verify those testimonials.
Most outsourcing vendors are working in different time zones. Ask them if they have people working in US time zone, and give preference to vendors who have people working in your time zone.
Local contact number
Does the vendor offer a local US contact number? If not, remember that long distance calls are expensive.
If the vendor accepts only wire transfers or western union money transfers, beware!
Ask for a credit card payment option (at least for the initial stages of work), so if things do not work out or if there is no service delivered, you retain the option of a charge back.
Reporting and support
Ask your potential vendors about the frequency of reports and the type of customer support (email / instant messaging / telephone) they offer. Email is the most common form of support. However, make sure that the vendor has a turnaround time of 24 hours for email support. Some vendors offer instant chat support. If so, ask them about their availability.
Most outsourcing vendors have people working in two or three shifts and hence should offer you some form of support in a US time zone. Although not many offer voice/telephone support, a vendor might include it for escalated problems.
In addition to the points above, agencies should also consider the following before outsourcing:
Private label service
Ask whether the company offers a private label service, so that reports sent to you are already branded in your company name. This saves you time and efforts inserting your logos and other brand elements. Also ask if your clients can get email support in your brand name. While this may not be possible for one or two projects, it should not be difficult to get this support if you are outsourcing multiple projects.
Confidentiality and NDA
It's very important to sign a confidentiality and non-disclosure agreement to prevent your clients being targeted directly if you stop working with the agency. Include a clause which says that the company cannot target your clients for a period of two years after the expiration of the agreement and that this clause stands even is you stop working with the agency.
Experience working with other agencies
Ask if the company is working with any other agencies. There is lot of difference between working with an end client and an agency, especially because of privacy and confidentiality issue. Although It's difficult to get references of agencies (especially because of non-disclosure agreements), try and get references of agencies which are in another part of the world, for example, if you are based in the US, ask if the company is working with any Canadian, UK, or Australian agencies.
Define the scope of the work
It's very important to define the scope of the work or the tasks you want to outsource, to make it easy for the other company to understand your requirements and deliver on those.