How to determine if two lines are parallel/ almost parallel? I have $2$ lines in this form
Line $1$: $(x_1,y_1)$ $(x_2,y_2)$
Line $2$: $(x_3,y_3)$ $(x_4,y_4)$
I want to detect if the two lines are parallel or almost parallel.
My idea is to if the angle between the two lines is $\leq$ some threshold angle (like $10$ degrees or so), then they are almost parallel.
But don't know how to compute angle between two lines.
Please help me with the equations or if there is any idea to detect almost parallel lines.
Thank you.

The others have responded on how to find angle between two lines.
Difference between slopes or angles of inclination of two given lines to x-axis .. that is what you want to control:
$$  \tan^{-1} \frac{y_2-y_1}{x_2-x_1}-\tan^{-1} \frac{y_4-y_3}{x_4-x_3} < \frac{10\, \pi}{180}$$ 
The smaller it is, the more parallel the lines are. If zero, they are completely parallel. 
Taking tan on both sides seems to bring it to an algebraic form, but I think it is really not better for numerical calculation.
