You’ll need a loop (a small magnifying tool, check out Justin’s workshop tour to see what he uses) and a bright light source. Take the opal in question, hold it to the light source and look through your loop. You’ll need to check the edges of the opal for a glue line; this line will be straight and you may be able to see the glue itself.
Some completely natural black opals can have a straight line between the color bar and the potch although it will never be perfect. Natural opal will have some areas where the potch and the color bar bleed into each other, even slightly.