Part of identifying a business defendant is determining who is authorized to accept service of your summons and complaint.
You'll need to provide this information to the process server so she can properly announce service during the delivery, as well as properly complete the proof of service form you'll file with the court. Court clerks routinely reject proofs that do not include the names of persons authorized to accept service.
Who to Serve
That depends on the business structure:
For sole-proprietors, service must be made on the proprietor only.
For a corporation, the agent for service (see below) or a company officer (CEO, Secretary, etc.) will do.
Limited liability companies, partnerships and associations also designate agents for service, but you can also serve a "managing member."
If any of these specific people are not present during the time of service, a "substitute service" can be made on someone apparently in charge of the office. This is probably where many attorneys get the idea you can just serve "anybody at the office" and don't bother telling the process server a specific name and title of a person authorized to accept. Remember, the process server must tell someone who the summons is for. Afterwards she has to mail it to that specific person.
Note in the case of sole-proprietors you're suing an "individual," so a few diligent attempts to get him or her personally served must be made before substitute service is acceptable to the court. A good process server will know this, and California's proof of service form includes these options for them to complete.
Where to Serve
Corporations and LLC/LPs must designate an "Agent for Service of Process" to accept legal documents if their company is sued. This can be a person like a member of the company, an attorney, or a company like LegalZoom or Corporation Service Company. But other people are also authorized to accept (see above).
Once again, you'll find this information using the California Secretary of State's online business search tool. This also shows addresses where to serve the business. Notice the "entity" vs. "agent" addresses. The entity is where the business itself is located, but the agent may be somewhere else.
You can serve at either the agent or entity address, just make sure you're not serving the agent at the entity address or an officer at the agent's address. Know the difference and be clear in your instructions to the process server.
For sole-proprietor defendants, you would locate them as any other individual. If they're doing business under a fictitious business name (FBN), the first place to look for their address would be in their FBN registration with the county clerk.
How to Serve
Be sure to correctly mark the capacity notice on the summons.