When is it not worth filing a lawsuit over an unpaid business invoice?

The clearest warning sign is when you genuinely can't answer the collectability question, meaning you don't actually know whether the debtor has any assets or ongoing business activity that a judgment could realistically be collected against. Filing costs real money in court fees plus your own time investment, and a judgment obtained against a company with no assets and no successor business to pursue is effectively just paper, legally valid but practically worthless.

Before filing anything, it's worth checking the company's current status with the Secretary of State, searching for any liens already recorded against it by other creditors, and confirming whether the business appears to still be actively trading. Building that collectability check into your process before filing, rather than after winning a judgment you can't actually enforce, saves considerable wasted time and expense.