How to tell if swim aids help?

There is a thread on benefits of using snorkels in swim training. I have seen debates in the past on effectiveness of various other swim aids - fins, pull buoys, tempo trainers, etc. The list of available gear is long. My question is - If I am using some piece of equipment as part of my training, how can I objectively determine if it is helping? If my times improve it may be due to other factors such as more training sessions per week, better designed sets (such as specific sets instead of "garbage yards"). Again a long list of possible factors. Love to hear some thoughts on how you determine if something is helping you. Can it be objectively determined with some reasonable method that does not require tons of measurements and equipment? Is it good enough that you feel subjectively that it helps, and that provides more motivation to work a little harder? I don't discount that factor. Sometimes if you believe it helps then it helps. Wondering how everyone else approaches this.