OK, Sorry to jump in, but I want to clarify some of what has been said. (All you PhDs out there, please don't cringe. I am going to water this down in a big way).
Waves are perturbations in the normal easterly flow in the tropics, and they DO originate over Africa. Sometimes they can make it a long way around the globe, but my undersatnding, from what I have read, is that they do not serve as seeding for tropical cyclonees far beyond the eastern pacific.
Their genesis is due to something called the Mid-Level African Easterly Jet. This is a band of strong easterlies that is at its most mature during the northern hemisphere summer. Without getting technical about it, it results becasue, well basically becasue of the sahara desert. The differential heating between the Sahara and regions to its south create a dynamic where by mid-level easterly winds are strong over a narrow area..the winds of the jet can drop off sharply as you head north or south from the core of the MLAEJ. Such a change in windspeed over a north south direction is horizontal wind shear. (We often talk about vertical wind shear when we talk about tropical cyclones--this is different).
The horizontal wind shear actually creates some "spin" in the air currents--this spin--or vorticity--is what gets a wave going.
So waves do originate form Africa.
Waves are a different animal from the ITCZ that Pojo mentions, though she's right--they often do enhance convection in the ITCZ or stay attached to the ITCZ, or even impact the ITCZ by drawing it north or south, but there are times and place where there are no waves and the ITCZ is still there (or waves separate from the ITCZ). For instance, during the southern hemisphere summer, the ITCZ is there (though south of where it is now)--but there are no tropical cyclones. One reason that some researchers suggest that this is the case is becasue in that season, there are no waves to provide the "spin" to get a tropical cyclone started. Anyway, my point is that waves are/can be separate entities from the ITCZ. The ITCZ always exists--and does not cause, nor is it caused by waves.
How do we identify waves? Yes, satellite imagery is one way. If you hang around the talking tropics forum, you will see a number of examples of folks discussing waves. Since the MLAEJ actually can get cranming over inland Africa, we can often see the convectrion/storms assocaited with them long before they come out over the Atlantic. Once they are over the water we can usually either see what's called an "inverted-V" signature in the low cloud field or, as Pojo points out, convection. Just to clarify, though, not every wave fires off substantial convection. Sometimes all we get is the inverted V.
A second way to detect waves is by surface and upper air observations. Meteorologists rely on the pattern in surface and upper winds in assessing where a wave is if all else fails. There are of course observations and soundings taken in western Africa, and the cape Verde Islands (
http://www.nhc.noaa.gov/stations/08594.shtml is an example of the kind of analysis.), as well as buoy reports.
And, yes, there are of course mariner reports (ship reports) that can add to the information--with reports on pressure, wind direction, etc.
Hope this helps. I wish more people would ask questions like this. See how it gets us talking?
WJS3