how to detemine if it is a dicot or a monocot?

The roots and leaf veins.  The vascular bundling differs, too, but I would ID them by the leaf veins and roots.

Roots are adventitious and the leaf veins are parallel in monocots .

Like the previous answer-er said:  You can ID them by the veins on their leaves (parallel veins on monocot leaves and netted veins on dicot leaves).

But there are three more ways that you can determine this if it is a monocot or dicot plant:

In flowering plants, the corolla (crown leaves - the coloured leaves of a flower) petals are in multiples of    three (i.e. 3, 6, 9, 12...) for MONOCOTS and in multiples of 4 or 5 for DICOTS.

You can also determine this by anatomical analysis - if you make a cross section of the stem of the plant you're studying and the vascular bundles are arranged in a cylindrical pattern just a small space from the epidermis or if it's fused (as one cylinder) then it is a DICOT plant.  If the vascular bundles are in now pattern whatsoever, then it is a MONOCOT plant.

If you observe the cotyledons in seadlings - when there is only one, you're dealing with a MONOCOT, if there are two, it is a DICOT plant.

:-)