How would I know if I'm lifting from speed or momentum? If you lift quickly won't that automatically create momentum? unless if you stop when you bring the weight down.

If you are lifting from momentum, some movement unnatural to the lift is occuring. When doing a lift with speed, you are simply doing it with more explosion. As goergen1 said, you want muscle fibers that come into play when lifts are done quickly and powerfully...

To elaborate a bit, the difference between an explosive lift and one that relies on momentum is that in an explosive lift you are (usually) following strict form and tempo and in a momentum lift you're (usually) not.

Here's a quick hypothetical situation outlining the difference in the two approaches:

You're bench pressing, you have the bar off the rack and you're holding it at arms' length.

Explosive lift - You slowly lower the bar to your chest over a period of 2 seconds, you pause for a second to ensure that all motion has been completely arrested, then you push the weight back up to arms' length as fast as you can. You allow the bar to rest there for a second before beginning your next rep. You do all this with perfect form.

Momentum lift - You drop the bar down as fast as you can, allow it to bounce off your chest and shove it back up as fast as you can. You immediately begin your next rep. Your form starts off good but mutates into something that barely counts as a bench press between the start and finish of your rep.

These are two exaggerated examples, one at near perfect tempo and form, and one using nothing but momentum to lift with poor form.

However, given the two approaches outlined, it's really quite commonsense which one is a fuller (and safer) development of strength.

Or, at least..you would think it is. Commonsense really isn't all that common.

I understand now, thanks for explaining!