You just have to look at a lot of genuine coins to develop an eye for it.
That's pretty much the same answer you'd get if you asked how to spot
fakes.
Usually with tooling the wear pattern is not correct.
That means that the high points are not (or don't appear) as worn as they should be.
What's usually
tooled are the
laurel wreath, where individual leaves are no longer distinguishable,
the hair and folds in the garment. The result is often a great contrast of amazingly preserved detail (which has been
tooled in) and an otherwise pretty worn coin. These details are the first that are worn down.
Plus the
tooled parts just look unnatural. It's not always easy to spot and takes a while.
If you suspect tooling, just compare with other (untooled) examples on
Coinarchives.
Usually after tooling the
tooled parts (or the whole coin) are repatinated, because tooling is basically
scratching the coin
and scratches are easily visible. So that's another thing to look for.
Andreas