Is there a way to get the amplitude of an audio clip sample in C#? using clip.GetData?

Not really. Even PCM files, which contain amplitude data in the range -37268 to 32767, and clip.GetData don't really help. Instantaneous maximum amplitude is not the same as perceived loudness and tiny blips can distort what you or your code think of as amplitude.

The thing is that we hear different frequencies at different levels of perceived loudness. You have to boost a 100Hz signal by about 30dB for it to sound the same perceived loudness as a 2000Hz signal. They could both have the same amplitudes in numerical terms but not the same loudness to our brains.

Compressed files such as FLAC and MP3 would need to be converted to PCM, something that you really don't want to get into, even if it did work, which it doesn't.

The best thing to do is to listen in advance and perhaps analyse with a suitable tool (the Open Source Audacity is as good as anything else for this) and come up with subjective measures.

If you're worried about excessive levels, try using Unity's Audio Compressor