Can you tell me a rational way to validate startup/project idea?

You're going to get the usual answers, which will be different flavors of find & talk to users. I will offer an unusual answer.

This isn't 2009 anymore. All the low-hanging startup, app, and SaaS fruit has been snatched up. You can't just "make cool things" and look for people to buy them. There's too much noise now. Too much competition. No one cares.

If from the outset you don't already know who you're selling to, how to reach them, with the certainty that your offer solves some pain-point, and with the certainty that people will actually pay for it, then you're better off just buying lottery tickets. Probably have better odds of success that way. And the only way to have that certainty is to already be knee-deep in whatever domain it is that your app is a part of.

If you can afford to optimise for your "Recall rate" over your "Precision" as such(e.g. your current time or number of opportunities/ideas is high), then go one step further than the Mom test and what people here say:

You need to listen to the diversity of what people think your solution will do and enable and you want a clear plural answer. If your problem is "urgent" and the solution is "obvious" the way people think it will work is too segmented, you can still have something that fails on unit economics because too many people have their expectations broken. (Learnt this from Jason Cohen). Really you need to talk to at least 10-20 people and try different questions, different approaches. Some ask about the problem. Some review paper designs with. Some, ask them to sit down and show you how they do the task. Generally very few ideas will be both an urgent problem people would pay for that people ALSO think will work and output in a fairly uniform way amongst the stakeholders.

In my personal opinion (and if you read some blog posts and books from people who are in the startup business), the best option is a basic MVP with the most critical idea-related features. This serves to prove your concept. Then, concentrate on marketing it (disregarding that it is not per-fect). And look at how people react. Don't count how many will react, but what their reaction is. This will give you an idea where to polish further.

Ask potential users for money before you build anything.

If you are solving a problem people are willing to pay for, people will pay for the possibility of a solution.

Good look.

1. Have a problem yourself in a field you know deeply

and/or

2. Talk to potential users and buyers, they are not always the same

You can do this without building or spending money

Validate what? That it can be a business? The rational way to validate that is to try to get customers.

Validate that it's worthy of building? That's different. Maybe it's a great idea the world needs, but with no real path to market.

Ask yourself the right questions.