Most of the evidence of Hezb'allah involvement in the Syrian civil war has been anecdotal, or from the opposition.

But the US government claims it has evidence the terrorist group's deep involvement in the war.

New York Times:

The United States accused the Lebanese militant group Hezbollah on Friday of a deep involvement in the Syria government's violent campaign to crush the uprising there, asserting that Hezbollah has trained and advised government forces inside Syria and helped to expel opposition fighters from areas of the country.

The American accusations, which were contained in coordinated announcements by the Treasury and State Departments announcing new sanctions on Syria, also accused Hezbollah of helping operatives of Iran's Revolutionary Guards Quds Force in training Syrian forces inside Syria. A Treasury statement said Hezbollah's secretary general, Hassan Nasrallah, had overseen these activities, which it called part of the Syria government's "increasingly ruthless efforts to fight against the opposition."

The accusations went beyond previous American claims about Hezbollah support for Syria's government. They seemed designed to counter Obama Administration critics who say the White House is not doing enough to back the Syrian opposition. They were also part of an effort to further draw attention to the alliance of Hezbollah and Iran, which American and Israeli intelligence officials have sought to portray as a subversive collaboration  that has not only destabilized the Middle East but has been implicated in terrorist violence elsewhere, including a deadly bus bombing of Israeli tourists in Bulgaria last month.

Nasrallah is caught between a need to stay neutral in Lebanon where pro and anti Syrian factions clash regularly, and helping his second biggest ally in the region survive. Hezb'allah's assistance  was described this way by an American official:

The group's deep familiarity with the Syrian landscape makes it a nimble and effective military partner," the official said. "Even though at current levels its assistance probably won't change the outcome of the conflict, it's prolonging the fight and contributing to the deaths of innocent civilians."

Some FSA soldiers have described Hezb'allah fighters acting as snipers, picking off civilians in the street, but that apparently hasn't been confirmed. Still, any help they give Assad makes it that  much harder to dislodge the Syrian president.