Swim - how do I know I'm getting better

Warmup good...

Do a 30 minute swim.  See how far you go.

Train for a couple months, repeat.

I'm no expert but:

1) I would measure your time against a few select distance (short, medium, and long) as use those as a tool to see if you're getting faster. Tracking time will also help you figure out if your endurance is growing.

2) As far as efficiency goes, the best way I know of is to count the number of strokes its taking you per lap and then either remember that or add it to your lap times to create a "composite" score. If you compare this over time, you'll be able to tell if your stroke efficiency is growing. The less strokes more lap = more efficiency per stroke.

Hope I helped a little bit

Efficient technique is a subjective thing, but how many strokes do you take for a 25 or 50?

 For estimating fitness, I like the 3x300 pace test. Do 10-20 minutes of easy warmup, then 3x300 free on :30 rest. Keep a hard pace, but a pace that you can hold for all three parts. (So if you swim 6:00 for the first, try to keep the 2nd and third within 10 seconds of 6:00.)

Repeat the test every 6-8 weeks or so. You should be able to hold a faster pace each time (Diminishing returns will set in.)

John 

The testing you should do all depends on the distance you are training for. I am prepping for an OLY and a Half so my testing is generally 1000m or 1500m TT. Do this once a month or so and see how it goes.

 Focus on staying steady with even splits for each 100. I generally end up faster on the first 500 and last 500 and a little slower int he middle 3rd