A good rule of thumb is that human nerve endings, whose sensitivity is logarithmic, can just barely tell a 10% change in a side by side comparison. For example, press on a bathroom scale until it reads 10 lbs, then lighten pressure to equal 9 lbs. You can only just feel that there is a difference in pressure on your hand from that or when you go back again from 9 lbs to 10 lbs Indeed, you may almost shake that much. Now repeat the same test on a postal scale using your finger and ounces instead of pounds. Same thing happens. You just barely feel a change from 10 to 9 and back again.
Now imagine that instead of telling a change from 9 ounces to 10 ounces on a scale in real time, you had to tell how a 10 ounce pressure felt one day, then tell it from a 9 ounce pressure felt years later. That's what detecting change in spring set by feel of the trigger or of cocking force is like. It's just beyond human nerve resolution as the means of instrumentation. You may be able to tell a 2 lb trigger from a 3 lb trigger that way, but a 15-20% change over time will simply go unnoticed by your sense of feel. You need a scale. Also, the cocking and trigger mechanisms will tend to wear in with use, so you are also faced with detecting a spring change from a mechanism friction change, and even the scale won't distinguish the two.
So I think removing the spring and measuring how much force it takes to compress, as Bart described is your best bet. Bart's board method works, but you can use a board and a couple of furniture clamp to press one against a scale. Just as long as you have the length inside the bolt to stop compressing it at, you should get a good result.
Now imagine that instead of telling a change from 9 ounces to 10 ounces on a scale in real time, you had to tell how a 10 ounce pressure felt one day, then tell it from a 9 ounce pressure felt years later. That's what detecting change in spring set by feel of the trigger or of cocking force is like. It's just beyond human nerve resolution as the means of instrumentation. You may be able to tell a 2 lb trigger from a 3 lb trigger that way, but a 15-20% change over time will simply go unnoticed by your sense of feel. You need a scale. Also, the cocking and trigger mechanisms will tend to wear in with use, so you are also faced with detecting a spring change from a mechanism friction change, and even the scale won't distinguish the two.
So I think removing the spring and measuring how much force it takes to compress, as Bart described is your best bet. Bart's board method works, but you can use a board and a couple of furniture clamp to press one against a scale. Just as long as you have the length inside the bolt to stop compressing it at, you should get a good result.