In monocot flowers, the number of petals is 3 or a multiple of 3. In dicot flowers, the number of petals is 4 or 5 or a multiple of 4 or 5. For example, a black-eyed Susan has 21 petals, which is a multiple of 3, so it is a monocot. A wild rose has 5 petals, so it is a dicot.

Here’s a visual:

In addition to that:

Flowers
Monocots tend to have flower parts in multiples of 3.
Dicots tend to have flower parts in multiples of 4 or 5.

Leaves
Monocots tend to have parallel venation.
Dicots tend to have net venation.

Roots
Monocots usually have adventitious roots.
Dicots usually have tap roots.

Stems
The vascular bundles of monocots are usually spread throughout the cross-section of the stem.
Those in dicots are usually spread to the outside.

Cotyledons
In theory, this is the best way to tell the difference between monocots and dicots. Monocotyledons, have one cotyledon and dicotyledons have two. However, unless you’re a botanist, it’s going to be somewhat difficult to look at a seed to determine the number of these.

http://condor.depaul.edu/sjost/gph205/notes/flower-petals.htm

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