In addition to evaluating the business (great answergreat answer), consider the potential payoff.
If bonds pay off in the 5-10% range, the S&P500 has averged 10.5%. You should be expecting a payoff of 15-20% to invest in something riskier than the stock market. That means that if you invest $10k, then in 5 years you'll need to get out $25K (20% returns over 5 years). If you get less than this much in 5 years, the risk-to-reward ratio probably rules this out as a good investment.