How do I know if there's no liquid under the keyboard of my Macbook Pro Retina?

You're right to be concerned. It sounds like you got really lucky, but you aren't completely out of the woods yet. Given what you spilled, I assume it was a thicker paste and the keyboards on Macs are surprisingly liquid resistant. It's not advertised as such because then people would think it's okay to spill on them and it's not. If the liquid got inside, which in your case it probably didn't, the thing you have to worry about is corrosion. Small amounts of liquid mixed with electricity corrode and build up minerals that eventually short out components. If you want peace of mind, take it to a shop and ask a technician to look for spills. Unfortunately, the Apple Store isn't really an option as when you tell them you spilled they will follow the procedure and tell you that your warranty is voided. Look for an Apple Specialist though and some might even offer cleaning services. If you're comfortable doing so, you could remove some of the keys and see if there is residue under them. If so. you can clean it off with alcohol wipes. Good luck! It sounds like you're probably okay but if you want to be safe it might be cheaper to get it checked out now than if there is a problem down the road.