Identifying chalkies, especially from pictures, can be difficult. Even for experienced collectors there's a certain amount of guesswork and luck involved. The Virtual Chalky Museum has a brilliant FAQ to help with identification. A model with any of the following characteristics may not be a chalky while conversely, a model without them might be chalky. If your model ticks more than one box, there's a better chance of him being a chalky. Characteristics to look for include:
Models and pictures owned by Sharon Walbridge and Damaia and used here by permission
White "halo" around rubs & speckled "roaning" effect to coloured paint
Models and pictures owned by Damaia and Deana Sprague and used here by permission
Underside of hooves thick and messy, sometimes with ridges or embedded wooden bits & speckled "roaning" effect to white markings - markings are typically an unusually stark white
Model and picture owned by Deana Sprague and used here by permission
Pooling or drips in flat areas or on undersides
Classic chalkies appear in both types - Base-coat and chalky plastic. Base-coat chalkies are the result of Breyer using cheaper coloured plastics and/or regrind plastic during the oil crisis. Models would be sprayed with a base-coat of white paint before having the body colour applied. This base-coat results in stark white markings, and frequently rich shading and sometimes an unusually matte finish. Models also appear a bit 'thicker' than usual as a result of the extra layer of paint.
The chalky models below are likely base-coat chalkies. They are shown with their non-chalky twin on the right for comparison.
Models and pictures owned by Deana Sprague and Heather Forrest and used here by permission.
Models and pictures owned by Damaia and and Heather Forrest and used here by permission.
Models and pictures owned by Sharon Walbridge and and Heather Forrest and used here by permission.
Models and pictures owned by PolarVrtX and and Heather Forrest and used here by permission.
Most often base-coat chalkies will be from the 1970s, but there have been a few larger-scale modern runs intentionally designed as such. The 1998 chestnut-going-grey G1 Seabiscuit is the only modern base-coat chalky Stablemate I've heard of. Both chalky and non-chalky versions of this run can be found.
Chalky Version: model and picture owned by Sharon Walbridge and used here by permission. Non-Chalky Version: model and picture owned by Heather Forrest.
Like base-coat chalkies, chalky plastic models are the result of oil-crisis experimentation. These models do not have a white base-coat, the plastic itself has a chalky finish. These type of chalkies are difficult to tell from base-coats and some collectors dismiss them as not being true chalkies. Chalky plastic is more opaque than the standard plastic, and in Stablemates tends to be more brittle, and has a 'crunchy' look to it (anyone remember candy cigarettes? This plastic always reminds me of these).
Super-white, very matte Stablemates popped up during the mid to late 1990s, around the same time as the translucent models. The plastic is very stark white and has that 'crunchy' look to it. Rightly or wrongly, I consider these models to be the modern version of chalky plastic chalkies.
Many thanks to Damaia, Deana Sprague, Sharon Walbridge, & PolarVrtX for providing pictures of their lovely chalkies!