I am using a P-channel MOSFET as a switch in my design & I have connected the drain to the load. The load needs 0.8mA of current. How can I decide whether the MOSFET is conducting that much current if I connect the source of the MOSFET to 3V ? The MOSFET used is BSS84 from Fairchild & the link for BSS84 datasheet is given below.
I think this is the graph of your datasheet that gives all the information you need:
So assuming this circuit
V_GS is -3V which allows I_D to be up to ca. 0.5 A.