1.. Know how (within a hundred dollars) much insurance and property tax will cost.
2...If you are not paying cash, know how much your financing is going to cost and what rate you can get.
3... Know (within ~$50) the rent for a comparable unit in the region.
4... Know what type of upkeep the property requires (lawncare, etc)
5... Know what utilities (water, etc.) you are expected to pay as landlord.
6... Factor in property management cost. (If you plan to manage yourself, value your time. I suggest 8% of gross monthly rent as a starting number).
7... Factor the cost for repairs & vacancies. I liberally apply 10% of gross monthly to both.
Add your monthly expenses and retainers (maintenance and vacancy funds) together.
Add your monthly gross rents together.
Subtract expenses and retainers from gross rents. Multiple the figure by 12. This is your net income. Divide your net income by the total cash out you are expecting to pay. That will be your "cash on cash" return.
Or download a handy spreadsheet tool or find a hand property return on investment calculator on the internet!