1. Location: facial, facial sparing, palmar, other locations
2. Spread: bullseye, centrifugal, centripedal, forehead first, head-to-toe, cheeks first
3. Appearance
4. By rash onset-fever onset interval
5. By pre-rash history, weird symptom
6. By patient
7. Others in DDx

Location: facial

• Acne vulgaris.
• SLE: butterfly rash, women.
• DLE: sharp marginated, on ears, lower lip, lower eyelid, nose. 
• Sarcoidosis: red-brown papules on eyelid, nostril rim.
• Rosacea: flushed, perhaps pustular. 
• Photosensitivity.
• Tuberous sclerosis: angiofibromas. 
• See also Slapped cheeks.

Location: face spared

• Scarlet fever.

Location: palmar

• Palmar erythema (cirrhosis).
• Meningococcus.
• Gonococcus.
• RMSF.
• 2° syphilis.
• Erythema multiforme ['target lesions'] (HSV, others).

Location: other locations

• Ring around scalp: tinea capitis, aka ringworm.
• Facial cosmetic area: cosmetics.
• Single dermatome: shingles.
• "Christmas tree" on back: pityriasis rosea.
• Glove or shoe distribution: allergic reaction.
• Between finger digits: scabies.
• Diaper area: candida.
• Gentials and lips: herpes.
• Groin, down upper thigh: tinea curis.
• Buttock and leg: Henonch-Schonlein purpura.
• Shins: erythema nodosum [infection of fat layer] (OCP, pregnancy, sarcoid, strep, drugs).
• Foot: tinea pedis, aka athlete's foot.
• Legs: erythema ag igne (heat rash).
• Extensor side of elbows/knees, scalp, lumbosacral: psoriasis.
• Flexor side of elbows knees: para-psoriasis, inherited eczema.
• Hands, feet, mouth: hand-foot-mouth.

Appearance

• Psilver Pscale over Psalmon Pskin: Psoriasis.
• Honey crust: impetigo.
• Rashes in all stages: chicken pox.
• Like measles but finer, pinker: rubella.
• Creamy white plaques, scraped to show raw skin: candida.
• "Active edge": central clearing, but border of lesion shows scaling: ringworm.
• Pinpoint petrochial: CMV.
• Clear center "marginosum": rheumatic fever.
• Central depression [punctum] in papule: molluscum contagium.
• Purple, Peripheral, Pruritic Polygons: lichen Planus.
• Dark blue edge around purulent ulcer: pyoderma gangrenosum.
• "Carpet tack" when remove skin: DLE.
• Cafe-au-lait:
  • Neurofibromatois
  • Tuberous sclerosis

By rash onset after fever onset interval

"Really Sick Must Children Must Take No Exercise":
Days after fever onset, that rash appears:

1: Rubella
2: Scarlet fever/ Smallpox
3: Chickenpox
4: Measles [with Koplik spots 1 day earlier]
5: Typhus, rickettsia [variable]
6: Nothing
7: Enteric fever [salmonella]

• Roseola: rash appears when fever disappears. 

Spread: bullseye

• Lyme dz.

Spread: centrifugal

• Typhus.

Spread: centripedal

• RMSF.

Spread: forehead first

• Rubella: forehead to face to torso to extremities.

Spread: head-to-toe

• Measles: both appears, and disappears from head to toe.

Spread: cheeks first

• Fifth disease [erythema infectiosum]: "slapped cheeks".

By pre-rash history

• Ate chicken: salmonella.
• Bit by tick: RMSF.
• Bit by flea: typhus.

By weird symptom

• Chorea: rheumatic fever.
• Condyloma lata: syphilis.
• Koplik's spots: measles.
• Gross splenomegaly: EBV [others also].
• Strawberry tongue: scarlet fever.
• Fluoresce in UV light
  • Yellow glow: pityriasis versicolor.
  • Tinea capitis, aka ringworm.
• Nail pitting, onycholysis: psoriasis.

By patient

• Born with rash: ToRCHeS:
  • Rubella.
  • CMV.
  • Herpes.
  • Syphilis. 
• Child:
  •  Listeria
  •  Measles
  •  Mumps
  •  Ruebella
  •  Etc.
• Very sexually active:
  • Gonococcus.
  • Herpes.
  • Neisseria meningitis.
  • Syphilis.

Others in DDx

• Contact eczema.
• Atopic eczema.
• Drug reaction, esp. penicillins.
• Allergies.
• Heat rash.
• Histiocytosis X.