Since this related to religion and specifically judaeo-christian mythology, I can answer with a Bible quote (Luke 6:43-45):
43 “For a good tree does not bear bad fruit, nor does a bad tree bear good fruit.
44 For every tree is known by its own fruit. For men do not gather figs from thorns, nor do they gather grapes from a bramble bush.
45 A good man out of the good treasure of his heart brings forth good; and an evil man out of the evil treasure of his heart[a] brings forth evil. For out of the abundance of the heart his mouth speaks.
This is actually a principle applicable outside religion as well. If the actions of the aliens/angels make it apparent that they are trying to trick you, then you will know. If this does not happen, you are losing nothing by assuming they really are angels. They either are functionally different from angels or not.
Additionally, from theological viewpoint God is both omniscient and omnipotent, this sets a limit to possible consequences of being fooled. God will not intervene to stop as from being fooled, but having fallible humans be (once again) fooled by false divinity will not actually have any impact on the divine plan for mankind. If God actually exists, no aliens can fool him. As such that the aliens look like angels is largely irrelevant, we should still judge them by their actions.
Moreover as somebody noted, if they really are angels, we find them because God wants us to find them, and it happens because God want to tell us something directly. I think we can trust God not to mess it up, we will get the message. We will of course be free to ignore that message, but we will get it, it will not depend on being able to tell angels and aliens apart.
As an added note, most popular concepts of what angels are like and what they are about are pretty much nonsense. So even angels looking and acting like we expect of angels would probably be putting on an act to ease the communication.
In summary, treat them with the respect they deserve, judge them by their actions not by their appearance, and pay very close attention to whatever they have to say to you. These are all true regardless of whether they are advanced aliens or angels from Heaven.