check whether my launch files are written correctly or not.
Depending on what you want to check though, there's roslaunch-check. In my team we've been using it for awhile on Gitlab CI. While it's very nice tool, its error output (esp. on CI) may be unclear. I have some knowledge that I may have not yet shared publicly so ask again if you run into issues.
roswtf, which I'm a fan of, AFAIK doesn't come with .launch file checking? What's already available is more for static and dynamic checking of the system's state and doesn't include the checking for config files like .launch files IMO.
That doesn't mean you cannot implement a check using roswtf. You can write your own plugin. I'd usually recommend opening a pull request to the core library, but since you're on ROS Melodic, for which I don't think the maintainers take any new features, you'll be unlikely able to have it merged into the official core library though.
Is there a way of analysing the results of roswtf
AFAIK roswtf was originally intended to print the result to stdout without any API, so what you get on stdout might be everything (I wish I am wrong but haven't found a way to programatycally interact with its output). I think I've tried to add API but I can't find a PR. Ayways, I'd answer as no to your question.
