Final answer:
Smoke stacks are not a key indicator when looking for plastics customers, as they are related to industries with emissions from burning rather than plastics manufacturing.
Step-by-step explanation:
The key things to look for when driving around to find a plastics customer are various indicators that a company is involved in the plastics industry. Silos might be used for storing plastic pellets, company signs could indicate the type of business, and rail cars could be used for shipping plastic goods or raw materials. However, smoke stacks are not typically a relevant indicator, as they are more associated with industries that produce emissions from burning, such as iron foundries or mills, rather than plastics manufacturing, which involves molds and other production techniques for creating items like take-out trays, foam packaging peanuts, and disposable cups.