Go to a Mass in Latin or another language
The best place to go to is obviously a Traditional Latin Mass. The Priestly Fraternity of Saint Peter (FSSP) and the Institute of Christ the King (ICK) offer the Traditional Latin Mass at many places in Germany.
The second-best option is to try a Mass in a foreign language. In major German cities, and even in some smaller towns, there are many Masses offered in foreign languages. It is less likely that the problems plaguing the Church in Germany are present in those congregations.
Another option is to find a Latin Mass (Novus Ordo Latin Mass). These Masses seem to be more solemn than regular Masses.
Look at the parish website beforehand
You should check out the website of the parish before you plan to go to Mass there. Check the site for rainbow flags, clown Masses, etc. Another warning sign could be if “pastoral assistants” (or people with similar-sounding titles) seem to feature a very prominent role in the parish.
“Family services” and first Holy Communions should be avoided
On many occasions, one can see so-called “family services” (“Familiengottesdienst”) being advertised. At these Masses (or perhaps “services”), lay people are sometimes given a very prominent role, for example preaching or doing silly children’s activities.
First Holy Communions are supposed to be a really joyful occasion, however, in too many places it is used as an opportunity to make the Mass as childish and “modern” as possible.
Additional tips
Like in many other countries, in many churches lay people distribute Communion in violation of Church law, which states that they can only do that if it would take an unreasonably long time for the priest to distribute Communion.
If one wants to go to a Mass in German, one could go to a Mass with a choir or music by a classical composer. The more solemn the music, the less likely it is for “pastoral assistants” or others to include activities during Mass which are inappropriate.