Calculate MOSFET Forward Voltage Drop
You estimate it with the charts on page 3 of your datasheet.  Specifically figures 3 and 4.

Figure 3 is telling us that the on resistance at 75 C is 1.4 times what it is at 25 C (the y-axis is normalized).



Figure 4 is telling us the resistance between gate and source is going to be around 20 mΩ at 12 V.



$$20 mΩ * 1.4 = 28 mΩ$$

Using Ohm's law:  $$5 A * 0.028 Ω =  0.14 V$$

Keep in mind these are rough estimates based on the charts.