I've been through several acquisitions and a few IPOs. If the company is acquired, you should contact the CEO and the CFO to ask the price per share at which the company was acquired. If you have vested options, they should send you a letter letting you know whether your options are "in the money" should you choose to exercise them (your exercise price is lower than the acquisition price per share). If the company has an IPO, then look up the price of the shares and figure out if your shares are "in the money" (your exercise price is lower than the IPO price) and read through your options agreement and the company's S-1 filing to find out if there is a mandatory holding period before you can sell any shares you may purchase through your options.
In an acquisition scenario, you are either exercising or not. In an IPO situation, depending upon your exercise price and the number of options you intend to exercise, there may be tax consequences you'll want to take into account and timing of exercise. If you have any other questions, please feel free to contact me.
One thing you need to know is if the stock your options convert into is registered stock. If so, you can sell it right away. If not, there is a holding period (6 months, I believe).