You asked how an appraiser would estimate market rents - typically they will contact both owner/landlords and property management companies and will do a "rental survey" to get a sense of what a particular property type will rent for in a neighborhood. They place most of their weight on recently rented properties because until something is actually rented - just a rental listing is of limited value.
It is better to think of the potential rent as a range and not as a fixed number - some of the things that create variability in the rent is if the landlord pays for some or all utilities, if the tenant is expected to yard maintenance or if the landlord contracts that out. Rent for the same type of house might vary by over $100 a month if the tenant is paying all utilities and responsible for taking care of the yard over one where that is included in the rent.
I think West Sacramento can be a very good investment neighborhood especially since much of the city is in close proximity to Downtown Sacramento.
I use two methods in Sacramento county the Section 8 housing guidelines show 2 bedrooms at a maximum of $1046 per month for a house but non section 8 rentals are generally at $850 per month. It really depends on the demand in any particular area. If you are purchasing a property you can ask the appraiser to do a rental survey which they will charge you for and will give you the market rents for area. ie they ask the neighbors what they pay for rent and look on craigslist.