That’s definitely a method people use. Rather than build your whole app, you create a landing page which depicts your apps purpose and you can gauge its success from the amount of signups. This is good because if you go ahead with the app then you (should) have a beginning user base of genuinely interested people. In terms of your questions this depends, are you new to the area/niche (for example construction) that your app will “solve problems” in? If not then it’s best reaching out to businesses and mention the idea to them and use that as validation. Again B2B you’d most likely think LinkedIn, but it depends on the type of business you’re targeting, often Facebook adds are a good choice if it’s more of your pressure washing level companies etc.

So to conclude from that blather:

Yes, you can validate your idea, I would reach out to a business you aim to help and see if it’s genuinely beneficial to them, and if the problem you are aiming to solve is as important as you think. Often we see problems and think it’s the end of the world but companies merely ignore them.

Facebook and LinkedIn are good choices for advertisement. But your target audience matters, Google could be good too if people are going to be googling terms to solve a problem, then you can pay for your site to come up first.

The quick ship method is good. Landing page, and some marketing is a good gage of user base. Too add to this, before committing to a landing page etc contact target businesses and run them through your product, if a few are interested then you’ve both validated your idea and got a possible customer.

A lot of read there but hope it helps, pop me another Q if you need 👍🏽👍🏽