What is this test?

I don't know what is this test's but I use some things:
1.Coordinate's shift to (0,0)
2.Checking:
Angle(a,c,b)==90
Angle(a,b,c)>=90
Pay Attention to:
VectorProduct(ac1,ac)>0, where c - is old C, c1 - new position
of C

Good Luck.