I can't post links as I've not posted enough - that means also images, so bear with me
First of all - all the maps(about topic) on eupedia are complete BS, that even wiki are better.
There is a mix up about slavic, that was created by proto-baltic-slavic term. Let me make it clear - there are NO proto-baltic-slavic - at best proto-slavs were balts. How they became slavs - that's an interesting question. Unfortunatelly - no one cares to think and acts like indian people, who still argue, that IE people did not invade India. FFS!
Balts have a lot longer history than slavs - all that area, where slavs originated, is full of baltic hydronyms - not slavic. The main earliest distinction between balts and slavs is that slavs have mix of
baltic and iranian linguistical base. That means - they were baltic, before influx of iranians. That all colerates to history of iranian influx and that slavs were mixed people. Let's look at SLAVIC area of origin in map(it is Belarus in center), where all the map of Dnipro river water basin is dominated by baltic toponymy, and there are actually NO SLAVIC toponyms - only few iranian:
s14.postimg.org/esxe8mqcx/hydronyms.jpg
As I see it, the task, that OP wants has to be divided in finding baltic groups first - because balts existed long before slavs. So, if you want to find what really makes slavs differ from balts, you have to find what was before slavs first:
upload.wikimedia.org/wikipedia/commons/thumb/8/8d/Map_Corded_Ware_culture-en.svg/640px-Map_Corded_Ware_culture-en.svg.png
Corded ware culture is what created Balts - they are same age. It looks like, that it also created or influenced Germanic hybrid culture with I1(and possibly R1b) and slight R1a mix(honestly, this is not the topic, that interested me much), that made germans IE speaking with 1/3 of unknown noIE language base.
This is the baltic toponymy map from the book of Marija Gimbuta book Balts(1963). As you can see, it does not include baltic toponyms in southern Finland and also newest ones in that might be in Germany:
g10.picoodle.com/ltd/img10/5/10/4/atasas/f_3p2w_a06_u7fu6.jpg
en.wikipedia.org/wiki/Toponyms_of_Finland:
"A few notable place names – such as a few major hydronyms Päijänne, Saimaa, Imatra and Keitele which are thought to be among the oldest toponyms – still lack a sound derivation from existing languages despite of different approaches."
Yeah, only Saimaa is baltic name for family. I actually found one more clearly lithuanianish like name in Finnish hydronyms, but Saimaa is quite big lake and southern Finland is known for corded ware, so no bother.
From wiki: "The term Pomerania Balts, or rather Western Balts, refers to Baltic people, who as early as the bronze age may have inhabited parts of the southern coast of the Baltic Sea, an area now known as Pomerania. According to Marija Gimbutas, the Baltic culture of the Early and Middle Bronze Age covered a territory which, at its maximal extent, included "all of Pomerania almost to the mouth of the Oder, and the whole Vistula basin to Silesia in the south-west" before the spread of the Lusatian culture to the region and was inhabited by the ancestors of the later (Baltic) Old Prussians"
Note: Lusatian culture was not slavic.
upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Vistula_river_map.png/444px-Vistula_river_map.png
Even though if Marija Gimbutas mentioned, that Vistula basin was Baltic(before Lusatian and gots moved in), it is not represented in her hydronym map.
There were some mentions that some name places in Germany might actually be with Baltic origin, but that's somewhat raises question about rather short inhabitation of those lands by baltic people, but I think, that with what we have it is already enough - with exception of Balkans, most of the lands where Slavic people expanded were baltic before.