How do I know if my sequences have the adapters?

I have been given some sequences paired end 2*300bp and i would like to know if they have the adapters. I know that i can use trimmomatic to get rid of them. But I'm not sure if they have the adapters or not before using trimmomatic.

If I use trimmomatic and they are already sequences without adapters. Would the sequences get modified after a retrimming?

Good morning,

You could use Cutadapt (tutorial) with the command
qiime cutadapt trim-paired

While these programs should only trim reads with adapters and not change other reads, it's still a good idea to compare your reads before and after trimming to see how many reads were trimmed and how much was removed with qiime demux summarize