Before investing in altcoins, you should look at the team working on the project. and also read the whitepaper or see the project roadmap.
I do all this things and thorough research before it guarantees me that this project in future will be successful. Also another thing is that will that coin have a demand in the next 1 year and what will drive that demand due to which coin will have a high value . Once this questions answers you then you are good to invest in that project.