Check the stats.
https://www.kickstarter.com/help/stats
Depending on the category funding success rates can vary from slim to pretty good.
Unlike indiegogo with kickstarter if the project is not funded you get your money back.
As far as red flags, common sense and some knowledge about what is getting funded.
If it's a new widget and they are looking to raise money for manufacturing you should have some idea of rough cost. If they are asking for too little they may get funded but will ultimately fail because they did not ask for enough.
Really depends on your appetite for what you will contribute. The lower coupla dollar tiers that are less than a Starbucks coffee who cares. If you are going to contribute the higher "founder" tiers in the hundreds or thousands do your research.