Is there a way to know what assistance if any, an inmate is already receiving? If someone wants to help a random inmate, how can they make sure it's someone who needs it most?

An inmate's "business" while incarcerated is private and they are afforded the same protection anyone on the outside would. Therefore, that information is not available. If you "know" this random inmate's name you can certainly use the services of InmateAid to convey that in the form of photos, greeting cards, magazines, etc.