For starters, look closely at the business model - how is the startup going to create value for customers, how (and when) will this translate into revenues, how straightforward is it for prospective customers to stumble upon this value and decide if it's for them?
Second, look closely at the risk model - in order to deliver the promise, where does the company need access to resources it doesn't have, what's the balance between the key resource categories of knowledge, labor and funding, and how does that balance change between idea and first markets, between first markets and traction, and between traction and scaling?
Third, look closely at what your arrival would mean - what's the balance between doing a job already mapped out, adding in experience that will flesh out important details and enable smart decisions, and architecting the business as it moves forward?
Last, look closely at what you want to get out of this opportunity - certain cash, experience and credentials for a future venture, a shot at a big payday, a role for the next ten years?
Now you're armed with the critical details. You're being sold an investment package: your time is the currency. So now you can see what value you'll create, and you can decide what level of risk you're happy to take on, and how you want to profit from your investment. So now, and only now, you are ready to negotiate.
From this point on, you need to find if there's a possible meeting point with how the founders are thinking. Do they see the value you will create as you do? Will they share that value with you in a way that works for you?
If the deal is as a commodity coder, you need to find a salary-driven formula that means you are comfortable you are being offered market rate - that what you're forgoing short-term is being recognized, accounted for and will be paid back to you in every success case. (That can literally be keeping tabs on a salary sacrifice amount and agreeing how it's to get paid back or canceled.)
If the deal is as an "all in" CTO sharing in risk and reward, you need to find an equity-driven formula that gives you a fair stake in the business going forward. In this case, the salary you want is just whatever you need right now to survive, and a note that says when and why you will (all) plan to move from "starvation rations" to "fair market rates."
Good luck. It's exciting to be in at the ground. And it's worth being ready to walk away unless there's a deal that's right for you.