Checks, letters of intent from customers who they've sold the pre-extant system to +, customer lists, etc.
Sketches are nice, too. I prefer charcoal as a medium and dragons as a subject. A good sketch makes a great complement to a check or LOI.
+ Happens all the time. Really.
* You're serious * You're a good salesman * There's a market for your idea
Getting to this point is a lot of work. It means you will have thought things through. It means you will have talked to your customers. It means you probably have mocked something up. You have probably invested a small amount of cash.
But mostly it means someone will pay for it which elevates your idea from "nope - waste of time unless you give me $20k" to "ok, we may have something here".
If you want to pay market rates (cash), I'd say show up with a mockup. If you want to convince a developer to join you for equity, show up with a sale.
Some years back, a guy named Bill and his buddy contacted a company making kit computers and said "we have a BASIC interpreter we'd like to sell you". The company said "a dozen others have said the same thing; we'll pay whoever delivers one." A few weeks later, Bill & pal delivered.
And thus Microsoft shipped its first product.