Lip slurs can be deceptive- when a player plays a lip slur, often, we think we are just “warming up”. And, a lip slur can help with the warm up. However, the other point of lip slurs is to work on flexibility. Often, flexibility refers to the “chops”. I like to think about the flexibility pertaining to the air stream- the tongue moving up and down in the oral cavity to direct and speed/slow the air.
Using Audacity, you can find inconsistancies in the air stream. You can also identify other inhibiting factors in the slur, like tongue use or chop based range.