To find a company's cost of borrowing, take all of their outstanding debts on their balance sheet and add them up. Then, find interest expense on the company's most recent annual income statement to find the dollar cost of debt over the period in question.
Finally, put the interest expense number over the total outstanding debt to find an approximation for the company's average cost of debt. This percentage should give a good sense as to how much the company pays to borrow money and should also shed some light as to whether the company is structuring its capital stack in a cost-effective manner.
For simplicity, understanding the average cost of debt and how to derive it is a good place to get started. If you want to get more specific, you can multiply the average cost of debt by (1 – effective tax rate) to find the after-tax cost of debt.