Official DevTools named LIFF Inspector is released.
I recommend using vConsole.
My way to check it. You can run localhost and then connect your mobile to the same wifi. Once you connected then you see your computer ip then use your ip to connect from line in mobile for example.
your computer ip: 192.168.1.18
and you run localhost port 3000
in line you can typing like this "220.127.116.11:3000" then the line's browser will promt and you can fix style if you want because liff and line's browser render the same