Just looking at it with your eyes you can't. But there are statistical analysis techniques used to find bias in random numbers, patterns in random numbers gener

Just looking at it with your eyes you can't. But there are statistical analysis techniques used to find bias in random numbers, patterns in random numbers generated using the same RNG and this is often used to demonstrate that some technique or generator is broken and cryptographically insecure. If a 128 bit number doesn't provide 128 bits of security then something in the generation of that number is broken. There are other comments in this thread that get into some nitty gritty details of this that I don't pretend to be an expert on.