Fentanyl or fentanyl analogs are sometimes sold as heroin. This has already caused many deaths in the US and Canada where those products are commonly used (you can google "heroin fentanyl overdose" to see for yourself), but it's quite a new thing in most of Europe.
The major issue with Fentanyl and analogs is that they are very potent and active at microgram rank (hazardous and undetectable for taste).
Technoplus, ASUD and Energy Control recently analyzed samples of #3 and #4 heroin, bought on the deep web from one of the biggest french sellers (with great feedbacks), and, as they thought, it was cut with ocfentanyl.
In April 2015, in Belgium, a young man died after taking a brown powder he had bought on the deep web and actually was ocfentanyl (http://www.eurotox.org/images/storie...ocfentanil.pdf.). In their June's newsletter, the British harm reduction project Wedinos said they had already analyzed six samples of ocfentanyl powder (http://www.wedinos.org/resources/dow...re_Issue_6.pdf).
The effects of fentanyl and fentanyl analogs are quite the same as those of heroin, except they last less longer and are a bit less euphoric (many people report feeling a strong buzz, but for only fifteen minutes), and therefore cause a strong increasing of tolerance and overdose risk.
Be careful when buying heroin on the deep web. Always take a small dose first and wait to see what happens. Also, if you're having troubles with some heroin, please say it in the product's feedback, even if the seller refunds you. That's the only way to protect the deep web from this.
There are no quick tests for fentanyl or fentanyl analogs. Although, if a product seems suspicious to you, it is possible to anonymously send a sample to the harm reduction association Energy Control that will make real analysis for 50€ (you can even pay in BTC) : International