Assuming you have several ideas, and you are trying to figure out which one among them to work on, I'd suggest:

1.Try to sketch the business model for the startup idea (referring to Business Model Generation - Canvas)

2.Get good at doing quick and dirty market sizing of opportunities

3.Check if you are passionate enough about the idea to potentially spend the next 5-8 years of your life in making it successful

Based on a collective evaluation of the above three, you might be able to zero in on an idea to investigate further (e.g. building a prototype, customer development, etc.)