Site probing in this case refers specifically to the art of locating a cosmic signature generated by the system, containing content as per below list. Similar mechanics can be deployed towards other objects and for now we’ll stick with “just” site probing.
There are the following types:
- Ladar
(Gas cloud mining sites - Gravimetric
(hidden asteroid belt mining sites - Magnetometric
(salvager or analyzer module is required to complete) - Radar
(hacking module is required to complete) - Unknown
- Wormholes (entry or exit point that leads to another star system in either known or unknown space)
- DED Rated Complexes (1-10)
- Unrated Complexes.
Understanding 
All objects in space has a Base Signature Radius. In the case of Cosmic Signatures, Beacons have been placed with a Signature Radius. It is in fact those beacons you are scanning down when locating sites and they are providing the information about the site (name).
When locating sites, you can use either combat probes or core scanning probes. Core Scanner Probes are preferred for site probing due to having a higher sensor strenght and a smaller pinpoint radius – which can help concentrate the effeciency of the probes.
Sensor strenght ranging from 40 to 45 points
Base Scan Range down to 0.25 AU
Base Maximum Deviation of 0.125 AU
Making them ideal to pinpoint Cosmic signature sites.
Sensor strenght ranging from 20 to 22 points
Base Scan Range down to 0.5 AU
Base Maximum Deviation of 0.25 AU
Thus making them a little harder to us, but due to having a larger scan sphere they can monitor a much larger area.
Since the equipment you have available to use is based on two variables, thet third variable is hosted by the target you are trying to pinpoint.
Maths
The formula is CA [Base Signature] X [sensor strength] / [Radius]
Base signature comes from the beacon. Each type of site (beacon) is categorized into bands, depending on the system. High, low, null or wormhole space each have different base signature strenghts for different beacons: 1/5, 1/10, 1/15, 1/20, 1/25, 1/40, 1/60 and 1/80. The easiest beacon to scan down is the 1/5 beacon with the 1/80 beacon being the most difficult and require good skills and equipment.
Sensor Strenght comes from the probes and the ship and having a bonused probe, a bonused ship along with scanning rigs and scanning modules will greatly increase your chance of a succesful/speedy probing.
Radius is the part of the probe that can be adjusted by the user. As this is a user decided variable, this is the part that helps you scan down stuff.
Warping.
Once the reading reaches 100% you will be able to initiate warp to the beacon you scanned down. If you are warping into a site with the Zephyr frigate, no aggro will be initiated and you can then place bookmarks. If you use a covert ops ship to warp into the initial site, be advised that you may encounter debris or objects that can (and quite potentially will) decloak you.
Some sites can be warped to with distance (ie. warp to 100km etc.) and this can be useful to avoid being decloaked by the beacon, but it does not liberate you from being decloaked by other of objects.
Be prepared to move the second you land on the site and re-initiate the cloak.
Is it for me?
If this activity appeals to you, think about what kind of ship you want to sit in when you hit the end-game. Be advised that you will lose a few ships getting there and the rewards might be a bit hit-and-miss. But when you return to high-sec space with a cargo hold full of shiny loot though, there is no better feeling! (untill you lose that ship in a gatecamp heading there!)
While some sites require capitals to run profitably, the best sites also provide loot/salvage in excess of a billion ISK and cannot be done solo.
For the solo prober, its quite possible to still do high-value sites in known space or even some wspace sites. While running null-sites can be done safely, transiting null-systems carry a great risk – which is why a covert ops ship might be useful.