View your mosaic correctly, don't spend too much time with your work right in front of your face. Every so often, stop step back and look at it from a different perspective. If you’re unsure of how the mosaic is turning out then move back and squint your eyes. This has the effect of blurring the individual tesserae so you are just left with the image. If this looks fine then just carry on. If something catches your eye when you do that then the chance is you have a point of focus and you need to correct that.
Image copyright Zane Green