There are a few methods which can be employed to determine the sex of puffing snakes, each with their own advantages and disadvantages. I've done my best to describe my experiences and opinions of each herein, as many new keepers find it difficult to accurately sex puffing snakes, largely for lack of information abounding about them. As always, I make no claim to being an expert on the subject - this is simply a synthesis of what I've learned so far.
Probing has long been utilized as the most accurate method of determining sex of snakes by advanced keepers. It is usually quite accurate, so long as one has a basis for comparison between males and females, and can be performed quite easily with a bit of study and practice. In short, the process consists of inserting an appropriately-sized, blunt-tipped, and sterilized steel rod into the vent of the specimen and gently sliding it toward the tail until it meets resistance - in males, the probe will slide into the inverted hemipene of the specimen and descend several sub-caudal scales deep before meeting resistance, whereas in females, the probe will typically stop quite short.
Needless to say, it's a process which requires some finesse and care, and it is best performed with at least two people - one to hold the specimen, and another to do the probing. It can also be quite a challenge with large specimens, even with two people, and it's quite stressful for sensitive species, such as Spilotes. In one case, I probed an adult male sulphureus and the specimen responded with an increased stress level and refused to eat for weeks after the experience. For this reason alone, I really don't recommend probing, and certainly wouldn't suggest it for the inexperienced keeper.
All that said, it can be helpful in some circumstances. In my experience and after speaking with others who work with the species, males typically probe to a depth of 12 sub-caudals or more, while females typically probe to 6 sub-caudals or fewer.
A new method which has recently emerged to determine sex of snakes is by way of genetic testing. For the hobbyist, the procedure is quite simple - just send in a recent shed skin of the specimen in question, the company will analyze the skin to determine the sex, and the results will follow promptly. Though a relatively recent method, genetic sex determination is reported to be extremely accurate for colubrid snakes. It's also relatively affordable at approximately $15 per shed skin, and it doesn't entail any invasive or potentially stressful procedures for the animal in question. For the combination of these reasons, it is becoming my preferred method for sexing colubrid snakes.
Follow the link below to a company that offers genetic sex determination.
Many keepers familiar with the species agree that sulphureus' close relative Spilotes pullatus expresses a degree of sexual dimorphism - the condition where two sexes of the same species exhibit different morphological or physiological characteristics beyond the differences in their sexual organs. In the case of pullatus, the sexual dimorphism expressed is in the degree of keeling present on the scales, particularly in the posterior half of the body. Keels are the raised ridges which can be seen in the center of the scales of many reptile species. With some experience, one can get a good cursory idea of the sex of an adult puffing snake by examining the keels - adult males typically have far more pronounced keels than adult females. However, it's unclear how well this pattern translates to sulphureus, as large females often express heavy keeling. Additionally, because this method is subjective and best employed with comparative analysis, it is not the most accurate or approachable method of determining sex. It is also not applicable for neonate or juvenile snakes, which have yet to develop pronounced keels. For these reasons, I tend to approach examining keels as providing cursory evidence to be followed up and confirmed with the more accurate methods described above.
The keels are especially prominent on the posterior half of this large adult male specimen.