Most K-girls in Los Angeles seem to have a screener to make their appointments. What is the best way to make sure I bring a sufficient donation without possibly incriminating myself?

First of all, with only a few exceptions, the standard rate for agency K-girls in LA is $250 ($260 in OC).  If she is completely independent, i.e., does her own booking, the rates may be higher, but I haven't seen any lately over $300.   Thai and Chinese girls will often be less.   If you are new to the booker and need to be screened, don't mention money until AFTER they say you have been approved, and then always refer to it as a "donation."   If the girl has recent reviews on TER, the fee amount shown in the review should be current.  Its wise to always take an extra $50 or $60 in case you want to tip, or in case the fee went up since the last TER review.  I always have extra cash just in case of any such unforeseen contingencies.  A few times I have extended the session mid-stream, by calling the booker during the session, which requires me to have another hour's worth of cash in my pocket.  I have never had an agency girl go through my pockets or try to take anything from me, so IMO its safe to have extra cash on you at an agency incall.