P.S. It helps a lot that the guy already know he is displaces in both location AND time. Many of the location methods would flub spectacularly if one did not suspect a large temporal displacement.

In 500 BCE Rome was a small insignificant town in central Italy. Nobody had heard of it outside Italy. The Latin language was spoken only in Latium, a small region in Italy. (And it was Old Latin, quite different from the classical language we learn at school.) Knowing Latin would not help the tiniest bit in northern Gaul.

If he had his smartphone with him he could just check the GPS :-p

Basically he'd have to be an astronomer specialised in the historic changes to the positions of the stars, planets & constellations as perceived from earth or have in his possession a charged battery powered pocket computer or iphone with an app loaded (in it's entirety including the full database, no calling the cloud for data sets) on it that does it for him, there really is no other way to do it quickly (presumably you mean within a few days?) or (particularly when it comes to 'when' he is) with any degree of accuracy.

With particular reference to the appearance & shape of constellations that incorporate one or more of the nearer stars to earth ^

Given a (now) full reading of your specs for the situation & individual the answer is simply 'he doesn't'

@SJuan76, what GPS? There are no satellites in 500BC; he won't get a signal.

@Matthew note the emoty type text face at the end, of SJuan's reply, I think you must have missed it ;p

Northern Gaul is hard; it looks pretty much "general western European" -- could be northern Gaul, could be southern Great Britain, could be Germany, even Poland, even quite similar to Romania. Why not place him in central Italy or in Greece, which are quite easy to recognize from weather, vegetation, and the look of the landscape? (And please note than in 500 BCE, "France" is about 1500 years in the future.)

@Pelinore, "tongue stuck out" is an ambiguous emoji to begin with, and even if it was intended to be tongue-in-cheek, it fell flat for me.

Would I be right in thinking he is basically dropped in the middle of a forest, a VERY large, VERY dark one?

It seems the answer depends a lot on a piece of information: did he install Kiwix on his phone and download all its English-language content? kiwix.org/en

@Nemo the contents of his pockets have been specified, he doesn't have a phone.

If his time machine was like the one in the 1960s TV series "Time Tunnel" he will be inevitably drawn to geographical areas and times that are notable in human history (as taught in 1950s/1960s US schools). Maybe he'll encounter marauding Celts and be able to recognized some loot from sacked Roman cities.

@SpehroPefhany Better re-read. It'll be roundly a thousand years before sacking Roman cities is a thing.

@ZeissIkon Ah, probably mixed the signs up.

@Pelinore Oh right! I got confused because several answers mention smartphones.

@Nemo Even if he had a smartphone the GPS satellites that the smartphone senses to tell you where you are are not there yet.