Simplest way to tell is by the file size. This is also affected by the audio format (multichannel or stereo). If audio and length are the same then bigger file means more data means better resolution.
well, it depends, if you try to really compress a bluray, it can become ugly. but it should be still better than SD (unless you go crazy with compression)
SD is very low in terms of resolution. under normal circumstances, SD should be worse than bluray even if it is compressed
Simplest way to tell is by the file size. This is also affected by the audio format (multichannel or stereo). If audio and length are the same then bigger file means more data means better resolution.