Hi @AJ Shepard and @Jonathan Manacchio
I agree using a rent to sales price approach is a great first filter. Then once you have filtered down that have a decent ratio then focus on calculating the cap rate. It is actually a very easy calculation on a single family or small multi-family. For rents always do your own research and work with a 3rd party not tied to the transaction. Contact your property manager as they have this information readily available and rents are pretty standard for locations and the "class" of the building. Your insurance agent should be able to provide a high level estimate, factor in your property management fee (even if you self-manage), put in your vacancy %, get property taxes from county online, determine who pays utilities and the costs, factor in a maintenance budget, and a capex for large future items. I feel if you don't calculate the cap rate then you run the risk of being negatively leveraged in reality if you are using debt.
If the rents are above market proceed with caution or don't proceed (Use the low end of a rent range, don't sandbag but be conservative/smart). If rents are below market use that for your calculation knowing you can improve the cap rate by bringing rents to market making the investment compelling.