It is a tough question and one that causes users and potential purchasers of magnetic separators a great deal of heartache. All you want to do is pull steel cans out of waste being carried on a conveyor. So why is it so complicated?
Suppliers of Magnetic Separators, such as Overband/Cross Belt/Suspended Magnets, can sometimes end up quoting the level of gauss as a certain distance or the force density. In most cases, they may as well be saying that the magnetic force is equivalent to a hundred elephants as how can you measure gauss, force density or, indeed, elephants?
The best way is to keep assessment simple. Refer to a standard test piece (a 25x100mm steel bar is good) and say that you want that picked up by a magnet suspended at a specific height (eg 300mm) above the conveyor. Leave the gauss and force density problems to the supplier and then test the magnet once it is delivered. Simple, cheap but very effective!
Bunting has put together a short video which can be seen on http://www.magneticseparation.co/company/news/how_strong_is_your_magnet.aspx. There is also some additional information that will help you select the best magnet for the job and also know the strength of your magnet.