It depends on how much time I have. If I can I like to download stuff and look at it, but I have a lousy connection these days. So I just check the file and post the "File Ok" screenie.
Atom has discussed the compulsory screenie, and the bit that should check the zip.... but I think he has already mentioned that they are difficult to do.
If the people who post the map don't want to use a screenie, you can't really force them... but unless they are new here, they should know how to make one.
If they do know how to make a screenie and don't post it.... well it says a lot about the amount of effort they have put into the map