What makes one worthless: survivorship and look-ahead bias, absent transaction costs and market impact, and undisclosed multiple testing.
A high-turnover strategy that looks excellent gross frequently turns negative once realistic costs are applied, which is the first thing an experienced researcher checks.
The question to expect: "your backtest shows a Sharpe of 3 - what do you check first?" A strong answer runs the list above and asks how many variants were tried before this one, which is the question most candidates never think to ask themselves.