If you're doing it on a server the only way you could really do it is to do the /spawn command, because there is a mod that tells you if you're in a slime spawning chunk but it doesn't work on servers, alternatively the only other way you could do it is to mine down to between 16-bedrock and search the caves till you find some slimes. And then you could set up some sort of farm down there as there would be some slime spawning chunks in that area.
If you want to do it legit, you may have a lot of work ahead of you.
You need to make large hallways below layer 16, covering a lot of ground, until a slime spawns in one. When you find an area that spawns a slime, you want to clear it out as wide as you can, completely clearing the chunk (a chunk is 16x16 squares). Unfortunately, its not going to be quick or easy, unless you get lucky. Sometimes, a single large slime will 20 or so slime balls, so that helps.
This is a blueprint on how to make an effective slime farming room, once you find a slime chunk. Whats important is that you FIND slimes first before you make the farm. Just making a farm won't do you any good.