(I thought I answered yesterday, but apparently I forgot to press "Submit" )
for the servers that you don't own you could ask the owners what seed they used.
This is what I usually do.
but if they are like me and inherited the server they may not know. When the seed is specified in words and not numbers in the server.properties, and I did not set the server up originally, the seed might not be known.
The world seed is listed in the server log around line 30.
Some owners might not want that info given out too.
Possible, but (in my experience) not likely: all server owners to whom I asked were kind enough to spend time in finding it and telling it to me. After all, it isn't a privacy/security sensitive piece of info...