Casting in an aegis

Is there any indication of how powerful an aegis is before you try casting in it with out a tolken?
Can you even tell where the edge is without seeing your precast spells fizzle or trying to cast?

I'd say no, you could devise intellego vim spells to see how strong an aegis is or where the borders are but those spells would be hindered just like any other if they were cast within an Aegis. You could sneak down to the library and see if they have any lab notes then hope that what ever was cast is the same spell as the one you found notes for.

Aegis is a tricky one. Unlike every other Hermetic spell, it will affect you reguardless of penetration. It's actually more like an aura then a spell. Fortunatly, the spell description itself says "When approaching the border of an Aegis, a magical person (including a Hermetic magus) feels a slight tingle in the extremities."

As this is a fairly unique feeling, I suppose most Magi would associate it with an Aegis, so yes, you can tell where an Aegis starts/ends. There is no indication that you can tell how powerful it is, however a Storyguide could very well indicate such with his description if he felt like it (from "a slight tingling" to "scorching pain")

However a more reliable way to tell is cast an Intellego Vim spell, level based on the power of the Aegis, the more powerful the Aegis, the lower the spell you need.

If you're inside the Aegis, I suppose, a Storyguide could just tell you "all your casting totals are at -15" or could just keep you in the dark and tell you what happens when you try to cast a spell.

Which all boils down to what you and your troupe prefer and what kind of stories, or rather, you want to tell.