The turnover number is kcat (Vmax divided by enzyme concentration) and is measured in units of s-1.You would have to know the concentration of the enzyme in the cell (assuming it is an intracellular protein), which is difficult, but not impossible to estimate. You would need to measure the rate of the reaction, which may or may not be possible, depending on the situation. You would also have to measure Vmax of the enzyme in the cell, which is basically impossible because you can't control the substrate concentrations in such a way as to allow a Michaelis-Menten analysis.