Making Medium-hot Mexican Sauces
Tasting the peppers is absolutely the only way, short of a chromatography machine. This is especially true for jalapeños from grocery. This because, as stated on this site here and in other answers by myself and others, pepper spiciness can vary greatly even on the same plant. Accordingly, chiles mixed possibly from plants, even from different harvests or suppliers, cannot be expected to have any reliable consistency of spiciness.

My approach is to pre-dice and freeze in batches. I use baggies, but another approach mentioned on this site here.

NOTE: The applicable advantage to this approach is that if you cook using the chiles often enough to warrant this approach, the heat level will be more consistent batch-to-batch.



My preservation process goes something like this:


Purchase many fresh chiles at once depending on what you will use in 3-6 months. (I use 25 to 50)
Sample the heat of each pepper during the preservation process
Dice the peppers (de-seed beforehand if desired)
Segregate into three groups:


Mild / non-spicy
Killer-blazing hot
Normal (everything in-between)

Freeze into single-use sized baggies (~1¢ each, non-zip)


Place the smaller baggies into a larger gallon-sized zip-type freezer bag (~15¢ each) labeled with the spice level of pepper type, date processed, and heat level the contents.



Though I always keep seeds because I want the spice, this approach can be used with or without a de-seeding stage in the process. Another perhaps applicable note is that IME, without seeds, jalapeños spiciness falls within a narrowed range.

To reiterate in summary, one advantage here beyond preservation and availability is that as you have the three piles of diced chiles, the spice heat level of each batch segment tends to be nicely consistent.