What I find useful is to treat each altcoin as you would any company selling a share in the stock market. Look beyond the value of the coin in the exchanges and especially don't get distracted by how the price goes up or down (especially if the volume of coins traded is below the millions as those might be easily "pumped-and-dumped).
When I saw Ethereum 4 years ago, I realized that they were going to fill an missing piece of the blockchain platform. With today's alt coins (and ICOs), find out if they have a working product, or has the team been part of a successful product before? Are they solving a real need, or just creating a use-case which may not be addressing a problem (medicine or vitamins)?