How do you know if your project plan is actually realistic?

A lot of people manage projects in spreadsheets. Not because they don't know better tools exist — because spreadsheets are flexible, everyone can access them, and you don't need approval from anyone.

But here's the thing: a spreadsheet never tells you:

• Whether the timeline is actually achievable
• Who's overloaded before they start drowning
• What happens downstream when something slips
• The actual probability you'll finish on time

It shows what you planned. Not whether it's possible.

How do you handle this? Do you just do the math manually? Gut feel? Some tool I don't know about?

Been thinking about building something for this but curious how others solve it first.