Northern Cricket Frogs are small warty skinned frogs reaching maximum lengths of around 1 1/2 inches. All Cricket Frogs (Acris) are characterized by a triangle on their head between their eyes, so this is not a useful characteristic for distinguishing the different species and subspecies among them. Even some Chorus Frogs (Pseudacris) tend to have a dark triangle between the eyes. There are three subspecies of the Northern Cricket Frog (Acris crepitans):

Blanchard's Cricket Frog, A. c. blanchardi
Eastern Cricket Frog, A. c. crepitans
Coastal Cricket Frog, A. c. paludicola

Note that in most field guides the subspecies referred to here as the "Eastern" Cricket Frog is usually called the "Northern". This change follows Crother (2000), where it is suggested that names be standardized in a manner that gives the species a common name that is different from its subspecies' common names. These photos are all Blanchard's Cricket Frog.

Reference:

Crother B.I. (ed.)  2000.  Scientific and Standard English Names of Amphibians and 
     Reptiles of North America North of Mexico, with Comments Regarding Confidence 
     in Our Understanding.  SSAR Herpetological Circular 29. iii + 82 pp.

This publication may be purchased from the Society for the Study of Amphibians and Reptiles (SSAR).