Are there different symbols or graphics to identify different types of GA fuels?
As Walker notes in his answer AvGas is typically noted by color when talking about telling it apart. In terms of aircraft markings for fuel this is governed by FAA FAR 23.1557 Miscellaneous markings and placards. and some more notes in this AC, 


  (c)Fuel, oil, and coolant filler openings. The following apply:

(1) Fuel filler openings must be marked at or near the filler cover with-

  (i) For reciprocating engine-powered airplanes -

    (A) The word “Avgas”; and

    (B) The minimum fuel grade.



These are fairly simple regulations to comply with so there is a wide variety of markings available, but the end results tends to be a decal looking something like this: 



(source)

Above ground storage tanks will also have to comply with local state regulations on petroleum storage, here are the New Hampshire regulations as an example. Generally this will require an NFPA style decal but pilots dont really use those to identify anything. The typical signage for 100LL is a red back, white fount, all caps AVGAS 100LL this is common on tanks, 



and trucks alike, 

(source)



(source)



Its also important that you as the pilot are present for fueling both to verify type and quantity. If there is any doubt about what is in a truck or at a pump some should be pumped into a canister and verified by color before fueling an aircraft.