How do you know if you should invest in a company? Think in terms of finance, ROA, ROE.. etc

That's a difficult question. I only invest in well known companies with long histories-HD, Coke, Southern Company, Verizon , etc. If this is for a long tern IRA or 401K, I would use Vanguard or T. Rowe Price. My favorites are Vanguard Wellington (65% stock and 35%bond) and T. Rowe Price Equity Income(86% stock and rest mixed.) What is your goal?

And sometimes, it is a guess, or a feeling. (although you can lose a lot sometimes)

A few of my friends, invested in a silly company, Tandy, way long ago, they became were Radio Shack, and my two friends became Millionaires, I thought it was too big a risk, oh well,

But yes, how much and what goal, what risk are you willing to take?