Without knowing much of the context or output, I'd say four initial things:

• remember the affordance in context (is it to be manipulated by mouse or finger, or both, and which of these better indicate that/both of those interactions?)
• similarly, remember the affordance must be 'visible' in the context of use (straight up, regardless of device, 4 and 1 throw up 'visibility' flags because of colour and sizing.)
• what are people used to? (for the output, in similar interactions, in similar contexts, etc)
• What can there be as a failsafe? Text input? (Sliders and Accessibility: Usability of sliders for users with disabilities)

And then test!

Without knowing much of the context or output, I'd say four initial things:

• remember the affordance in context (is it to be manipulated by mouse or finger, or both, and which of these better indicate that/both of those interactions?)
• similarly, remember the affordance must be 'visible' in the context of use (straight up, regardless of device, 4 and 1 throw up 'visibility' flags because of colour and sizing.)
• what are people used to? (for the output, in similar interactions, in similar contexts, etc)
• What can there be as a failsafe? Text input? (Sliders and Accessibility: Usability of sliders for users with disabilities)

And then test!