how do a cell differentiate b/w meiosis and mitosis ? i mean how do it get to know that it have to do one of them and not the other ?

I believe it is still very much debated in the research field. We know what the signals for starting mitosis are:

1) the levels of cylins and cyclin dependent kinases within the cell.

2) presence of the M-phase promoting factor (MPF).

3) Cell is large enough to divide and has the necessary nutrients (G1 checkpoint).

4) Ensuring DNA replication has occurred correctly (G2 checkpoint)

5) Density and anchorage dependence.

6) Some external signal triggering the movement to M phase (for example PDGF in fibroblasts).

 

I would hazard a guess that some, maybe all of these are at play as well as some more specific signals. Since meiosis doesn't occur in humans till after puberty, there is likely involvement of some of the sex hormones and other specific proteins that trigger the initiation of meiosis at the gonads. Some other proteins suggested to be important in meiosis are RME1 and IME1 (at least in yeast). RME1 is suggested to inhibit IME1 and IME1 is a positive regulator of meiosis. Similar proteins are likely conserved in humans.

 

Still is very much a black box and a hot topic in research.