Yes, you need a way to measure it, and there are potentially a number of ways to do it depending on your system, the signal modulation, and the receiver architecture.
e.g., if it is a PSK system and the receiver maintains lock, SNR can be estimated by taking statistics on the error vectors after bit slicing.
"Slow fading" refers to comparison with respect to Symbol Rate, SR.
There are Large and Small Scale Fading, see "Fading Manifestations and Mitigation Techniques"
Small Scale fading is a random variable, so you cannot predict SNR at each point in time, but rather know the distribution function, Rayleigh or Rician.
Got it. thnx Shafie
The SNR is a random variable in a fading environment, so have to think of it in terms of a distribution, from which you can derive summary statistics (e.g. mean SNR). In a practical implementation, one would typically estimate it from a pilot channel (where the transmitted information is known). For typical fading profiles used in simulations (e.g. Rayeligh) you can derive it analytically.