Is there already a budget for this kind of tools? Marketing? Sales? R&D budgets? etc. If there is - who makes a purchase decision and who will be the user? Try to find contact with these people, setup a meeting and talk to them to understand - how they are solving this problem today, what tools they are using? How much do they pay? What is the biggest problem with the current solution? etc.. - more you learn, more you validate your idea and partially market.

Try to have several such conversations. Then you can build a "sales" demo - it can be a presentation with slides, or it can be quick demo-prototype (it's possible to develop it in 1-2 weeks in some cases). Then you can setup a meeting with the most friendly potential customer from previous conversations and do the first demo run with them, then refine demo based on feedback and do it again

In Slack’s case, there were existing solutions that companies had paid money for like hipchat or IRC. So existing competition is one way.

If you find that a company is spending skilled time (translation:money) on a crappy internally-cobbled together verson of your product, even better.

Ideally, you need to validate your product on whatever channel you intend to use.

So if you expect to get signups mainly through your website, you need to test that channel as early as possible.

Eventually you'll be getting hundreds (millions?) of signups through your website. So getting just a dozen for your beta list should be easy. If it's not, then you need to either pivot or find a better channel.

But if your website isn't the way you expect to gain users, then try a different channel.

And I don't think Slack is a good example since Campfire and Hipchat were already in use many years earlier.

The reason to create a new category is because the current categories don't solve the problem for a specific target audience. When diving into such uncharted territory, due diligence should be performed from the founder's end. This is where the lean startup idea is coming from. It is hinting that go out and tell your potential users that they have an unmet need, how their needs are unmet, and how your solution will meet those needs. If truly the need is met with your new solution, then you will hear that and this is where you ask for commitment. The potential user said that you are meeting an unmet need and they are on board, then ask them to sign up. This can go beyond just a signup on a soon to be coming page. You can actually ask the user to pay upfront or commit to pay upfront and if you meet requirements XYZ then they will pay. A lot of startups I talk to had not even started to code, until they had two willing to pay customers. Our site https://www.waccal.com is a free site so we didn't ask for payment up front, but we did have a signup for beta page. From which we had over 100 signups and knew there was a need.

To add on further, we wrote an article about idea versus execution and it dives into this towards the end. Here is the link: https://medium.com/waccaler/idea-is-only-a-small-piece-of-th...

I do agree with afarrell's comment that there was already existing solutions in slack's market. Slack just did it better and met a lot of needs that were not met. Also, slack is a marketing machine, which can explain a lot of their traction as well.