How can an investor determine the efficiency of a company's working capital management?

There are a number of tools that determine how efficiently a company is managing its working capital, principally by looking at measures of inventory and cash flow.

Analysts and investors look at a company's working capital to determine its overall efficiency and financial health. Working capital is essentially the money necessary for a company to maintain its operations on a day-to-day basis. It is composed of a number of components, the three most important being:

• Accounts payable
• Accounts receivable
• Inventory levels

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https://www.investopedia.com/ask/answers/041015/how-can-investor-determine-efficiency-companys-working-capital-management.asp

Understanding the Cash Conversion Cycle

"The cash conversion cycle (CCC) is one of several measures of management effectiveness. It measures how fast a company can convert cash on hand into even more cash on hand. 

The CCC does this by following the cash as it is first converted into inventory and accounts payable (AP), through sales and accounts receivable (AR), and then back into cash. Generally, the lower this number is, the better for the company. 

Although it should be combined with other metrics (such as return on equity and return on assets), the cash conversion cycle can be especially useful for comparing close competitors because the company with the lowest CCC is often the one with better management."

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https://www.investopedia.com/articles/06/cashconversioncycle.asp