A mole on my arm has become darker and now has a black line running through it, its only small but i would like to know if it needs checking out?

See below: If you have noticed changes in the mole, it should be examined.

Changing mole: Any mole with visible changes, especially in color, size, symmetry, edges, should be evaluated by a dermatologist immediately.

Easily resolved: If you are concerned about changes in a mole, the best action to take is to be seen by a dermatologist, who is trained to clinically evaluate the mole, determine if a biopsy should be done, and actually do the biopsy. Useless to speculate otherwise.