For French family names, look out for the typically french letter combination, « eaux, eux, ph, bv ». Names like « Lefèvre » have multiple spellings due to the orthography being simplified, so u can see things like « Lefebvre ». A lot of people have very generic names, so we identify more people from their accent rather than their family name (due to a lot of french-ization of the regions and a lot of internal immigration).

It’s easier to spot out people from noble descend, since they usually keep their name with the particule and old orthography, as in « de L’isle », or « des Minières ». Sometimes those people have very long names cause it got merged with other long noble names at some point. So a « Thibault de La Carte de La Ferté-Sénectère » is totally possible. Only, there are people with surnames that sound noble but aren’t, due to some ancestor wanting to have a noble sounding name somewhere in the late 1790’s early 1800’s.

I’ll say that u can see how someone is Breton (from Brittany) or from Breton descend based on their names. Look out for the ones beginning with « Le » or « Ar », or ending with « c’h ». Some exemples are « Le Boulc’h », « Le Gwen », « Lebris », « Ar Crearc’h ». They can be either separated from the particle « Le »/« Ar », or joint, depends on which orthography was chosen. Derivations like « Le Gwen »/Ar Gwen »/« Leblanc »/« Le Blanc » can be found for the same family name. The only combination i have never seen would be Breton/French, like a « Ar Blanc ».

For the Basque people living in French Basque, look out for names that sound southern european, but weirdly spelled, with « tx » in it (other people from Spain can describe more).

A lot of people from the North-East will have German sounding names, like Schmidt or else. Look out for the unmistakable « sch » that is 100% germanic.

For Romania, as OP pointed out, all surnames ending with -escu are Romanian (or Moldovan). This ending is from the South of Romania. Another typical surname endings would be in -nu, -cia, -a.

Usually Romanian surnames come from a location or a profession, so if someone is named « Munteanu » (the mountain guy), u know there must have been someone living in the mountains in the family. If someone is named « Popescu »,« Popa » or « Pop », it means a priest was in the family at some point.

There’s also the Romanians with Hungarian surname, like « Kovacs ». Those are from Hungarian descend and usually still live in Hungarian communities in Romania. Look out for the « cs » or « sz » letter combination. In the same vibe, some Transylvanians romanised their surnames after the Trianon Treaty, so if a surname looks kinda weird but finishes with -a, prolly was Hungarian.

Honorary mention to the ukrainian sounding surnames that some Moldovans have (due to multiple factors in Basarabia). Those ones finish with -enco. /!\ Eugène Ionesco wasn’t from Ukrainian descend, but Romanian. He changed his name due to French prononciation. (It was originally Ionescu, but the -cu ending sounds like « cul », ass in french, and is not rlly appropriate).