I use time. The usual flow is like this:
- Day 1: exciting idea comes to my mind. I start to think a bit about why it would make sense to implement it, who would pay for it, etc. I do some investigations (e.g., how many competitors are out there? Would I ever come with something "better"?) and if things don't look good, I discard the idea... otherwise I let it stay in mind for a few days. Most of my ideas don't pass this stage.
- Day 21: if I haven't yet discarded the idea, I do more investigations. I evaluate potential competitors (if there are none, I check why). In general is now when I spend more time digging around. If I feel confident about the idea I tell it to people I know which I consider would be potential customers.
- Day 41: if I haven't yet discarded the idea, I start working on it. This could be a simple online website (just a landing page) where I track visits and the like. I do some marketing in places that I know people would be interested in my idea, etc.
- Day 61: if I haven't yet discarded the idea, I take it more seriously. It's now when my idea becomes a "serious side project".
Usually, 1 idea or so every 2 years reach the "Day 61" stage. Almost all my ideas get discarded because I realize at some point that "they don't actually make sense. Let's not waste time on them".
Note: I don't think one can "quickly" validate ideas. If you decide something in a matter of a few days... you are probably missing a lot of details (unless you have a whole team working for you): and most probably the outcome will be "wasted time". For learning, sure, it does make sense to "waste time" but I'm assuming here that the goal is not "learning" but to actually decide on what idea one should spend time on.