If you have difficulty referencing the Salter-Harris classification of fractures through the physis, remember the mnemonic "ME".
Hey you !! Who ME? Yeah, you, what kind of Salter-Harris fracture is that? ME stands for Metaphysis and Epiphysis. The SH-I fracture, we all know is through the physis without any involvement of the metaphysis or epiphysis. The SH-II fracture is through the metaphysis (M) and the physis. The SH-III fracture is through the epiphysis (E) and the physis. The SH-IV fracture is a contiguous fracture through the epiphysis, the physis, and the metaphysis (ME). The SH-V fracture is a crush injury of the physis.
See diagram of the SH classes: 
An SH-I fracture may be suspected on clinical grounds alone. The fracture line may not be radiographically evident if the epiphysis is not displaced. Tenderness over the physis should lead you to suspect an occult SH-I fracture in the region of the tenderness even if the radiographs are normal. This commonly occurs in a wrist injury where normal wrist radiographs may lead one to the pitfall of diagnosing a wrist sprain. If there is tenderness over the physis of the distal radius or ulna, a clinical diagnosis of an SH-I fracture of this area should be made. An SH-I fracture is only visible radiographically if the physis is widened, distorted, or the epiphysis is displaced.
Radiographic confirmation of clinical SH-I fractures may be made later during orthopedic follow-up by stress views or the presence of new bone formation along the physis approximately 7-10 days post-injury.
See SH-I example
This radiograph shows a tiny fracture of the ulnar styloid. The AP view is otherwise unremarkable. The patient had point tenderness over the dorsum of the radial physis. The lateral view shows a displaced radial epiphysis. On careful inspection, you can see that the radial epiphysis is not centered over the metaphysis. The radial epiphysis is slightly displaced dorsally with respect to the metaphysis. No fractures of the epiphysis or the metaphysis are visible. Since the fracture is strictly through the physis, this is a Salter-Harris type I fracture.
An SH-II fracture occurs through the physis and metaphysis (M).
See SH-II example
This radiograph shows a fracture of the distal ulna and radius. The radius fracture extends from the metaphysis into the physis. The physis appears to be slightly widened consistent with SH-II.
An SH-III fracture occurs through the physis and epiphysis (E). Since this fracture often involves the articular surface, this injury is more prone to chronic disability if anatomic realignment is not achieved.
See SH-III example. 
This radiograph shows a fracture of the distal tibia over the articular surface into the epiphysis and physis.
An SH-IV fracture is a contiguous fracture through the metaphysis, physis, and epiphysis. This fracture often involves the articular surface, making this a high-risk injury for chronic disability as in SH-III injuries.
See SH-IV example.
This radiograph shows a fracture of the medial malleolus extending from the inferior articular surface of the tibial epiphysis through the physis and extending through the metaphysis.
An SH-V fracture is a crush injury of the physis. This may be radiographically visible as a narrowing of the growth plate lucency; however, it is most often not radiographically visible.
See SH-V example.
This patient fell off a second story balcony onto her feet. The radiographs show several fractures within the body of the calcaneus. A Salter-Harris type V injury of the distal tibia was suspected because of the mechanism of injury. However, this type of injury is rarely visible on initial radiographs. The injury must be suspected clinically. Subsequent growth arrest of this area confirms the presence of the Salter-Harris type V injury.
Comparison views of the non-affected extremity may assist in radiographically diagnosing a SH-V type injury at initial presentation. Based on this comparison view, differences in the width of the growth plates may be evident. A complete obliteration or diminished physeal distance of the affected extremity confirms the diagnosis of a SH-V injury. However, even if there are no obvious differences on the comparison view, or if a comparison view is not obtained, or if both extremities are injured, the patient should be treated as a possible SH-V injury if the mechanism of injury suggests an axial compression along the long axis of the bone, and the patient exhibits tenderness along the physeal region.
References
Bachman D, Santora S. Orthopedic Trauma. In: Fleisher GR, Ludwid S (eds). Textbook of Pediatric Emergency Medicine, third edition. Baltimore, MD, Williams and Wilkins, 1993, pp. 1237-1238.