Empirical proof of randomness is impossible. It can be shown that the output of some generator tends to some distribution as the number of trials approaches infinity, but this does not eliminate the possibility of a deterministic process underlying the generator's operation that can be described by such a process.
To use your example of tossing a coin or a die, the process can be described macroscopically by a uniform distribution, but the system is in fact deterministic (again, ignoring quantum effects). This is called pseudorandomness.
A better example of a pseudorandom process is that of random number generators as used in computers. The algorithm takes a seed value and produces an output based on that seed that appears to follow a uniform distribution, but the algorithm is in fact not random at all, as giving it the same seed will produce exactly the same chain of output values.
I'm going to go and do something productive now