Bitfinex has now changed their fees and takes different fees from makers and takers. https://www.bitfinex.com/pages/fees

I assumed that a maker is one who adds a bid or ask order which wont get filled by an already open ask or bid order. However today I placed an ask order which had no corresponding bid, but it was charged at the taker fee. Maybe it was filled by a hidden order?

How are we supposed to know with certainty that our order will be a taker or maker order?