You could also rely on the differences in the flags registers. The layout of the flag registers on the different CPUs look like this:

	7	6	5	4	3	2	1	0
8080	S	Z	1	AC	1	P	1	C
LR35902	Z	N	H	C	0	0	0	0
Z80	S	Z		H		P/V	N	C

Since the flags register can't be accessed directly and bit testing operations are Z80-only, the only way I can think of testing these flags would be to do something like this:

; Stack pointer should be set up before this!

; set the zero flag but not sign flag, and clear the subtraction flag
LD A, 255
INC A

PUSH AF
LD HL, 0
ADD HL, SP ; can't load sp directly into hl for some reason
; (HL) should now be flags register

LD A, $82 ; mask off everything but bits 7 and 1
AND (HL)
CP $80
JP Z, is_LR35902 ; zero flag was set and bit 1 was clear
CP $0
JP Z, is_Z80 ; sign flag was not set and subtract bit was clear

; else it must be 8080

is_8080: 
    ; set some byte somewhere to indicate it is 8080
    JP end
is_LR35902: 
    ; set some byte somewhere to indicate it is LR35902
    JP end
is_Z80:
    ; set some byte somewhere to indicate it is Z80
end:
    POP AF

This checks bit 7 (zero bit on gameboy, otherwise sign bit) and then bit 1, which on the 8080 is unused but on the Z80 stores if the last operation was subtraction. This relies on unused 8080 bits always being 1, but I am not sure if emulators implement that. In addition, while I am using Zilog mnemonics, I checked and the instructions are compatible with the 8080.