T3 Boosting alts are very difficult to scan down. The T3 boats already have a reasonably small signature, but as well as that they often fit ECCM mods which make the scanning signature even harder to scan.
In saying that though, it is possible. In a recent patch (6 months or so ago I think?), they changed these fits to not be unscannable anymore. It still takes a long time to narrow it down though, as you have to usually scan right down to .25AU on the probe radius to get a %100 hit. Where someone is "soloing" and only uncloaking/boosting while they are actually fighting, then you usually only have a limited window, as these fights don't last very long. I guess you're sorta hoping for someone to leave their boosting alt uncloaked, and not watching directional scan. You're also a little bit more likely to be able to scan these down if they are in a bigger fleet fight, which tend to last a lot longer.
Some of these fits are very pimped, and can only be scanned down with :
a perfectly skilled scanning character. Just the skill names start with Astrometric, plus the Covert Ops ship skill.
using scanning implants. There is a set called Low Grade Virtue, which is expensive but helps a lot, these go in slots 1 - 5. There are other implants with "prospector" in the name which help too, which go in slots 6, 7 and 8. On the plus side, even though the implants are expensive, the scanning character will usually be very difficult to kill if he stays in a covert ops boat.
using a covert ops frigate, fitted with two Gravity Capacitor rigs, Sisters Expanded Probe Launcher (or the Tech 2 version), and using the Sisters Combat Probes.
Check out the eve online wiki page http://wiki.eveonline.com/en/wiki/Probing for the full names of skills and implants that will help.
The other thing that having combat scanning probes out will often do is cause the character to cloak the boosting alt back up again, losing them their link bonuses. While it won't net you a ridiculously sweet killmail, just stopping their boost alt from boosting can help a lot in some cases. Sometimes this may be the far easier thing to accomplish. A skilled player with a boosting character will often watch to make sure they only cloak when the scanning probes are closing in. Not everyone who can afford a boosting alt is a skilled player though... :)