How to check for sharpness in small LCD screens ?

Question

I have recently bought Macro lens(Tamron 90mm) and been experimenting with it, My camera is Canon EOS 1000D and it has only 2.5" LCD screen.

Immediately after taking the picture, How can one check whether interested area is in sharp focus (especially if you have shallow DOF and photographing bugs eyes, petal edges , pollen etc.?

I have seen people zooming in and check for sharpness, How much zoom-in can approximately tell that particular area is in focus? If we zoom in 100% naturally everything is going to look almost pixellated?

Objective of question is, What usually photographers do to quickly check whether image has come up sharp or not?

Answer

The LCD is great to check for sharpness on-the-field but you need to know your camera's 100% magnification ratio.

When you zoom-in into the LCD view, most cameras display a magnification ratio as X times from the fit-to-fill size of the LCD. So initially, it is one and when you get to say 4X, the image is magnified 4 times in both directions. As you keep increasing magnification, you will reach a point that depends on the sensor AND LCD resolution where the pixels are one-to-one. This ratio is usually between 8X and 11X on a modern DSLR.

If you go beyond that, which you can in models of the last few years (I cannot imagine why someone thought this was a good idea), cameras will interpolate pixels and start blurring the image which works against checking sharpness.

Check more discussion of this question.