Aura perception would let him know instantly if something is a shadow (or if it has an obfuscate rating higher than your Auspex), but it wouldn't let you avoid the dice roll penalty that nightshades give you.
Now, I don't know that Steeple would have the knowledge necessary to make any of these decisions but these are the following ways you could get rid of a nightshade summoned by a Lasombra (That I can think of with Steeple's toolset):
The nightshades, much like the shrouds and the tendrils, can only be summoned within eyesight. Out of eyesight the Lasombra can still command them to do a specific action, but will be unable to guarantee that they do exactly what they are supposed to. That is to say, if someone is running away and they leave your sight, but you have a shroud floating after them, you can make the shroud move forward, but you can't guarantee that the shroud will move at the same pace as the person, that it won't pass over them and continue moving forward, or that if the person turns around a corner the shroud won't turn the corner itself. The nightshades are sticking to your flesh right now but if you were to teleport again, unless the person commanding the nightshades has vision of you, the nightshade wouldn't be able to stay on your person, as they would no longer be attached, and would just swirl in place.
It's the same with the tendrils. Even if the Lasombra isn't within eyesight, he can still order the tendrils to attack and restrain whatever they are attached to.
Alternatively, all Lasombra shadows are extremely vulnerable to sunlight and fire. Steeple could set himself on fire (not recommended), or summon a fire elemental to cover his body in flames without burning him, both of these run the risks of rotshreck but would destroy the nightshade.
Steeple could also walk out into the sunlight (not recommended).