Removing ourselves from something like the internet where we are bound by what rules an ISP would use, a typical network would not be able to distinguish between false UDP and true UDP.
Think of UDP like a child yelling for their mom in a crowded supermarket. The child can yell and mom might or might not hear, but the child is going to keep yelling since they don't know if mom heard it. As a parent, you'll be able to differentiate your child from someone else's, but there are times when some other kid sounds just like yours. You could easily mistaken the yell of one child for yours. Since you and are child are separated, you don't have a 'handshake' to establish the child and mother in one conversation, it's just noise.
Since UDP is effectively just putting data on wire, there is nothing stopping you from spoofing the traffic -- you won't get a response back, but you can make the application think Alice is doing the talking.
In a real world, you have multiple layers of stuff which could easily identify that you're trying to send spoofed traffic and just drop it before it leaves your network segment.