Measure Wavelength of IR LED

Most of the detectors work with both 940 nm and 850 nm LEDs. The curve for the TSOP31238 is the first a saw with so much difference. Usually the sensitive range is more like 750 nm to 980 nm. So it is not that important to have the same wavelength as the original.

One possible way to distinguish between might be using a 850 nm LED as a detector - it should have rather low sensitivity to light from a 940 nm LED - mainly from the small amount of < 860 nm light coming from a 940 nm LED. You may still need two detectors to compare, so one with a normal Si photodiode and one with a 850 nm IR LED as a detector. The ratio of the signals from these two should be rather different.

I have not tested a 850 nm IR LED as a detector so far. Some IR diode use quite some tricks for better efficiency and thus these LEDs might not work that well as a detector at all. But chances are quite good for it to work.

DIY spectrometers should work in the NIR range as well. The main limit is the usually silicon based sensor. So sensitivity could be lower at 950 nm, but 950 nm have quite some intensity. The simple way is using a camera (preferably without IR filter - so more like cheap web cam). So getting one without a IR filter (or removing the filter) is the only challenge.