To kick off, I'll share with you what I do whenever an idea pops to mind.
1) Adwords keyword tool First I go to the Adwords keyword tool, to find out if people actually are looking for my idea/future product. You can check per keyword how many searches per month there have been done and if there's a lot of (SEO) competition.
2) Surveys I setup a survey for potential customers. In the survey I try to find out if my assumptions are right. I use this survey for step 3 and 4.
3) Run Google ads Running Google ads is a great way of checking my assumptions. I emphasize different benefits per ad. This way I can check what benefit appeals the most. The more clicks, the more important this benefit is, the more this should be the focus point of the product. The ad leads to my survey. Not so many people take the time to fill it in, but still it is some extra data.
4) Meet with potential customers Although I always find it hard to take this step, I know it's important. These meetings can motivate tremendously, can force to iterate my idea, or make me decide it was a stupid idea. The survey comes in handy in these meetings as well, btw.
So that's my viability-check. I'm wondering what you do to check viability. Looking forward to your ideas and advice!
ps. I'm sorry if I made some mistakes in my English. I'm no native speaker and I know that especially grammar tenses are not my forte...
