The first thing I would do is burn a simple ROM with something like (pardon my asm, it's been a while):
START MOVE.L #$0000,D0
LOOP MOVE.L ($DFF180),D0
INC.L D0
JMP LOOP
Now from memory, and it's been a long time so I've probably got the syntax/numbers wrong, but this would just increment the background colour, resulting in multicoloured strips all over the screen - if the system is working.
Too bad this is not enough. When the Amiga system is powered up the colour burst DMA is turned off, thus you won't see anything at all. So to see anything at all, the colour burst DMA must be explicitly turned on first.
Fixed code:
start: lea $dff000,a6
move.w #$7FFF,$9A(a6) ; All ints off!
move.w #$01FF,$96(a6) ; All DMA off!
move.w #$0200,$100(a6) ; Colour burst on!
moveq #0,d0
loop: move.w d0,$180(a6)
addq.w #1,d0
bra.s loop
Further, at this point the ROM appears in place of the chip memory until the ROM overlay bit is turned of from CIA... So in case you must check chip ram, you need to turn off the overlay first.
