Historically, there has been no simple, reliable way to measure if food contains active B12 for humans, for many reasons. Therefore, it would ideally be better to test B12 on humans - if only a human would show signs of B12 deficiency once his levels were low.

This article, from 1949, describes a method called paper chromotography, which combines several other methods. This may still be the best way to test plants for B12.

The organisms that have been considered most usable for B12 tests using microbiological assay methods are Ochromonas malhamensis and Arthrobacter Lochhead. Escherichia coli mutant, Lactobacillus leichmannii (AKA Lactobacillus delbrueckii) and Euglena gracilis Z-alga may all respond to inactive B12 analogues.

Liquid chromatography is a much newer method, but unfortunately, there are people out there - even vegans - who insist that there's no need to test any more plants for B12. They assume that B12 comes from the animal kingdom only, and that there's no reason to use or develop methods to measure B12 activity in plants. I'll just pretend that these people don't exist for now. All in all, there are extremely few tests performed on B12 in plants, and tests which documents the ratio between active and inactive B12 in plants are almost non-existent.

Re. testing B12 activity in the body:
The homocysteine test was for a period considered to be the best way to test B12 activity in the body. Some still insist that since there (to some degree) is a link between low B12 levels and high homocysteine levels - the low B12 must always be the (more or less only) cause, and the too high homocysteine level is the result, and that the remedy is to increase the homocysteine levels, hence the importance of taking homocysteine tests. But it's not always that simple. MMA tests seem to be a better solution for testing B12 activity in the human body.

Most plants that have been tested for B12 have been using methods that have been around for quite some time.

Here's an old article comparing some of the microbiological assay methods, of which Ochrmonas is the one that's considered best.

And - as one can see from the table on page 304, even if some methods are considered more reliable than others, they sometimes come up with relatively similar results. It's not possible to paste a table from a pdf file into a forum post in the current forum version, so I'll just mention a couple of examples from that table here:

Cow's milk wast tested for B12 both using the Ochrmonas method and the with Bact. coli. The mean result when testing with Ochromonas was that milk contained 0.31 mcg B12/100 g. With the other assay, the mean result was 0.27 mcg/100g. In other words: the method that is supposed to be most accurate because it is "capable of measuring cyanocobalamin specifically in a variety of crude extracts, some known to contain vitamin B-12-like compounds active for Bact. coli" found more B12 than the less accurate method. One reason for this may of course be that there isn't a 100% static amount of B12 in all milk.

The article also refers to testing dried meadow fescue silage for B12. The mean result when using Ochrmonas was 0.25 mcg B12/100g, and 0.12 mcg B12/100g when assayed with Bact. coli. In this example, the Ochrmonas result was only 50% as high as it was with the other method. These were only two random examples - there are 6 more examples in the pdf-file.

It's all a little confusing - and if you want more confusion , read up on MMA (see the link above), which explains why some people think that even MMA tests aren't reliable.

So -what to we do if there are no really reliable ways to make sure that we get enough active B12?

The best answers I can come up with are:
Don't assume that you get enough B12 just by eating healthy (this is true for non-vegans as well, although they are at risk of having a constant high B12 level, which is something we don't want...)

Remember that even if we would have lived - many centuries ago - in a non-sanitized society completely void of anything synthetic, we'd still need to deal with the fact that things like stress, lack of sleep, sugar and eg. coffee can disturb our B12 levels.

Since we actually know that today, substances in water, air, soil - and chemicals used when growing plants - are B12 antagonistic, it's a good idea to compensate for these losses.

It's also worth knowing that although one - if looking at eg. allergic the risk of getting reactions from high B12 intakes - easily may assume that it's safe for most of us take really high B12 amounts over long periods, there's research showing associations between high B12 level and some cancer types, increased mortality rate and more - so don't overdo it.