How can know if a ppa repository is a true repository or a malicious repository?

Question

when I try to install software like a Gimp in the ubuntu I find the following steps

1. Add this repository

2. Update

3. Install the software

But of course when I find on the gimp page I didn't found any repository, so my question is how can trust that it is an official repository?

For example, I found this repository for gimp:

    sudo add-apt-repository ppa: otto-kesselgulasch / gimp

But I cannot verify if this repository is fiable or if these binary files have not any malicious software

Is this technique a best practice, I think that if you install this and this source is a not a trusted source, you can take a virus or trojan.

Any idea?

Thanks