TI's Webench can probably do the rough estimate that you're looking for. Here is a link where you can input parameters on the right hand side of the page.
If you want to know more, read further.
Dividing the output power by the input power is the quickest way to measure power supply efficiency. One thing to keep in mind is that efficiency is a function of output voltage, input voltage, load current, and many other things. The following are the biggest contributions to losses in power converters.
- Conduction losses
- Switching losses
- Loss in magnetics
Calculating these numbers can get quite involved and aren't worth going into unless your really need to know efficiency ahead of time. The reason this gets really involved is because these losses are influenced by many variables / parameters, all of which need to be known or there is a rough estimate.
There are many good books on power converters that go over this information. One of the best I've seen is Switching Power Supplies A-Z.
edit: updating link