...Because they count the Omer after Ma'ariv each night? Obvious, but no.
...Because men go to work unshaven, and do not feel out of place? A good guess, but again no.
...Because you stop seeing weddings until after Lag B'omer (for the most part)? Still true, but not the best way to tell.
No, the reason that you know it's definitely during the Omer in Israel is when you see...
...random children collecting wood from building sites, corners, garbage bins - basically anywhere, as they prepare to build obscenely large bonfires on Lag B'omer.
We can't wait!