Photo Mechanic provides multiple ways to check for critical focus while previewing your photos.

The Loupe Cursor

From the contact sheet view, the loupe cursor is a magnifying glass that allows you to preview selected areas of photos at 100%. You can choose the loupe cursor from the menu or use the keyboard shortcut Z to toggle between standard and loupe cursor modes. 

Zooming from the preview window

In the preview window, zoom controls in the zoom panel allow you to specify how much magnification you would like on your previewed photos. Zoom enlarges both images in a 2-Up layout.

The zoom range is between 1/4  (25%) and 8x (800%) magnification. You can also use the keyboard shortcut of + and - to increase and decrease zoom levels. 

Just as with the contact sheet window, toggle between zoom and standard views in the preview window using the keyboard shortcut Z.  Grab and drag photos within the preview window while zooming.  

Courtesy Jason Hill Photography

Zoom to a specific portion of an image

When previewing without zoom, Command-select (Mac) or Control-select (Windows) on any point on the photo to zoom into that point of the photo. 

Lock and zoom

When viewing 2-Up photos, the Lock scrolling toggle (keyboard shortcut L)  drags both images at the same time.  For best results, use two similar photos and align the images before locking.