How do I contact social services for a suicide watch for someone I know?

You can't. Social services do not provide suicide watch. If someone is seriously suicidal, the only safe place is a hospital. Call a local psych hospital for more information.
See below. You can ask your local police department to do a well-being check. Also, you can take this person to the er, where s/he will have a psych. Eval. If a person requires a suicide watch, then the hospital is the safest place for that person to be.
Suicide watch. One important intervention to consider is a meeting of friends and relatives to tdevelop a plan to take turns in the presence of the person, with the latter asleep or awake and with relevant emergency phone numbers available. This is not a treatment but a stop gap until a more definitive intervention can be activated.