In addition to evaluating the business ([great answer](https://money.stackexchange.com/questions/3335/what-should-i-look-at-before-investing-in-a-start-up/3337#3337)), 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.