It's really very simple. Ask anyone who is hiring you to MSG you the
details of what occurred. Seek clarification on any part of the events
that seem unclear, then make your own decision based on that. If the
target later is assisted, then, like most mark do anyway, say you'll be
attacking them later, if they say "No, the contract was taken without
cause, and here is -why-," and it fits in with what the person who hired
you says (i.e., it fits, but includes a detail that that person left
out), it's fairly safe to assume that the contract is, in fact, illegal.
If it totally conflicts with what was said, then contact the person who
hired you again, clarify that if possible, and if later it proves that
the person assisting was, in fact, in the right, because the contract
was taken out without cause, then you've done all you could to protect
yourself, and ensure that you were only claiming legitimate PK, and any
issues against -you- likely won't stand, though the person who hired you
then will be smacked harder.

Basically, just do all you can to stay informed of why you're filling
the contract, and why someone else chose to assist, comparing the
stories from both sides, since if you do that, I strongly doubt any
issue filed against you will stand, since you have done all you could to
ensure that the person hiring has legitimate cause. The only reason an
issue should stick to you, is if you don't try and find out if the cause
is legit (which is the failing of most mark, currently, since they just
don't -care-, it won't reflect on them one way or another), if you don't
act sensibly when you have reason to doubt the legitimacy of the cause,
or if you misinterpret the PK rules when deciding whether someone has
legitimate cause or not, and so attack an assister illegally, when you
think it is legal.

Penned by my hand on the 9th of Glacian, in the year 395 AF.