This item appears much the same as any beaker, bottle, container, decanter, flask, jug, etc. It has magical properties, but detection will not reveal the nature of the flask of curses. 

It often contains liquid substance too, or it may emit smoke. 

When the flask is first unstoppered, a curse of some sort will be visited upon the person or persons nearby (it will subsequently be harmless). 

The suggestions given for the curse reverse of the cleric bless spell, and those stated for typical curses found on a scroll are recommended for use here as well, or some monster can appear and attack all creatures in sight, etc.