two pieces of circumstantial evidence
Normally, if Israel has undertaken a successful terror attack (such as those against Iran) it will either refuse to comment or a senior military or political figure will saying something like: While we refuse to comment, whoever did it did the world a favor.
In this case, Israel immediately denied responsibility.
The second tell-tale sign is that Israel never offers humanitarian aid to its Arab neighbors. During the Syrian Civil War the only group to whom Israel offered humanitarian assistance was its Islamist anti-Assad allies. Israel has never offered such aid to Lebanon, until today. Instead, it has rained down death and destruction for decades. For it to do so now is the height of chutzpah.
https://israelpalestinenews.org/breaking-israel-bombed-beirut-silverstein/