How do you know if a moment is clockwise or counter clockwise?

Refer to the diagram below.

The moment caused by F₃ consists of two components that one of them is counterclockwise and another is clockwise.

First of all, divide F₃ into a vertical component (red) and a horizontal component (blue).
F₃ and the two components form a 3-4-5 right-angled triangle. Hence,
the vertical component of F₃ (red) = 500 × (4/5) N
the horizontal component of F₃ (blue) = 500 × (3/5) N

Consider the vertical component of F₃ (red).
(Think, move vertically the vertical component of F₃ up to the right hand side of A.)
(This is shown by another red arrow.)
The lever arm length is 5 m and the moment about A is clockwise.
Moment of the vertical component of F₃ about A = - [500 × (4/5) N] × (5 m)

Consider the horizontal component of F₃ (blue).
(Think, move horizontally the horizontal component of F₃ to the right until it is under A.)
(This is shown by another blue arrow.)
The lever arm length is 4 m and the moment about A is counterclockwise.
Moment of the horizontal component of F₃ about A = + [500 × (3/5) N] × (4 m)

Total moment of F₃ about A
= - [500 × (4/5) N] × (5 m) + [500 × (3/5) N] × (4 m)
= - 5 × [500 × (4/5)] + 4 × [500 × (3/5)] N m