One of the great time-saving things about Nutshell is that you don't need to "convert" between prospects, leads, deals, opportunities, or any of the other terms people use to talk about their potential new customers.
With that in mind, it often still makes sense to be able to track how valid a lead actually is! In Nutshell, the best way to track this conversion from a prospect (someone who you're not sure is a lead yet) to a real business lead is by building the prospecting step into your sales pipeline. Check out these helpful articles:
If a prospect isn't valid, you would then cancel the lead. You can create custom outcomes to measure leads that are closing for this reason. You can even use Nutshell sales automation to automatically cancel leads if they remain in a stage for too long.
Use your funnel report [Nutshell Pro] to measure the success of your prospecting efforts in your first stage.
If you do need to track companies or people that are prospective customers but aren't quite valid leads yet and you don't want to add them to your pipeline, it also makes sense to add the "Prospect" company type or tag a person with "Prospect".