Many manufacturers have roadmaps available on their website for their product lines. Check this and make sure there's active development in the line and on the product you choose.
Find the revision date on the datasheet. Was the last update 10 years ago?
Are other distributors stocking the part? Do they have healthy inventories?
By looking at the company's website, press releases, etc., do you get the feeling that this is an area they are pursuing, actively researching?
For the most part I think simply asking the manufacturer should be quite reliable. EOLing a part without notice can give a company a pretty bad reputation. I don't think asking the distributor is better than the manufacturer though.
And don't get too hung up on product lifetime past a few years. This world moves fast. Unless you're designing legacy hardware, you probably won't need that chip a few years from now. You'd probably have a better product if you upgraded sooner. Design the physicals (enclosure) to accommodate some flexibility in PCB size/shape and then you're golden. Spinning a new PCB is relatively cheap and it makes your product better and cheaper if you take advantage of new chips that come out every few years.
