Context and word order. Here the information that "I was here" is treated as known so in English you would stress the "in front of you"/"before you" part (i.e. pronounce it stronger than the rest of the sentence). What would you rather say this way: "But I was here in front of you!" or "But I was here before you!"?
If you wanted to say this sentence with "here" stressed, that is, assuming the whole location you were at is unknown to the person you're speaking to, you would rather say "But I was here in front of you!" than "But I was here before you!" because saying the latter this way would make an illogical connection between place and time.
A similar effect (probably - I'm not a native speaker nor a very good speaker so if somebody could confirm, it would be appreciated) can be achieved in Czech by saying "Já jsem ale byla tady před vami" or "Já jsem ale byla tu před vami" with "tady"/"tu" stressed when the whole phrase means place and and with "před" stressed if the phrase means time.
TL;DR Watch the sentence stress.