You don't know and can't choose; it's random so roll dice
The alchemical item elemental flux (20 gp; 1 lb.) says
This flask of alchemical powder reacts to the elemental power in suli blood. Adding at least 1 hit point worth of suli blood to it creates an alchemical splash weapon. The splash weapon works like a flask of acid, except the damage is a random energy type (acid, cold, electricity, or fire). A DC 10 Craft (alchemy) check identifies what energy type the activated flask has. The activated flux retains its power for 24 hours before becoming inert material. The Craft (alchemy) DC to create this is 20.
The entire point of elemental flux seems to be its random nature.
The suli are descended from jann, genies that are mixtures of the four elements. Until the flask of elemental flux is activated by the 1 hp of suli blood—y'know, the very metaphorical essence of the elements or whatever—, elemental flux is just processed alchemical materials. When the suli blood activates those materials, though, the mixture becomes a randomly determined splash weapon. (Roll 1d8: 1–2 acid, 3–4 cold, 5–6 electricity, and 7–8 fire.1)
The elemental flux flask's possessor has no way of knowing what kind of energy damage a flask of elemental flux will deal beforehand, and, even after activation, the possessor must still make that successful Craft (alchemy) skill check (DC 10) to determine the elemental flux flask's energy type if he wants to know what kind of damage it'll deal when he throws it.
Even if you make the elemental flux yourself, you can't predict what'll happen when the suli blood's added. I mean, if you could, then other alchemists could, and, instead of making folks go through all this guessy-guessy stuff, alchemists would just slap labels on the outsides of their elemental flux flasks.2
But then we wouldn't have an interesting item anymore; elemental flux would be just another boring, predictable splash weapon like acid or alchemist's fire.
Keep it random. For some people random is more fun than efficient.
1 I hate d4s, but you could use them. I guess.
2 Assuming a campaign of non-jerk alchemists.