How well signposted are the EuroVelo bikepaths in Germany?
As there isn't streetview in Germany, it's hard to check. Could one realistically follow the signs for the EuroVelo cycle path (e.g. #6) through Germany?
It's hard to give a general answer for any EuroVelo bike path in Germany.
Last year I followed EuroVelo 2 (or EuroRoute R1) from The Netherlands to Berlin. It was reasonably well marked, but better in the more touristic areas than in the remote parts. A few times I lost the track and needed my guidebook and GPS track to find my way back.
My guess would be that over 95 percent of this particular route is well marked and easy to follow. I think the remaining stretches are still marked though, it's just that some shields are missing or hidden. I would definitely advise to bring a guidebook or GPS track for those cases.
In Germany, there are signs from Tuttlingen to Passau.
See this map for details.