It is hard to tell. In the United States, heroin is typically found in two forms: off-white powder (primarily along the east coast), and black tar (on the west coast), with fentanyl/analogue-laced samples predominantly being found in the powder form.
Because illicit, clandestinely-produced fentanyl is the same drug as pharmaceutical fentanyl, it is impossible to distinguish the source of the fentanyl involved in fatal overdoses by relying on post-mortem toxicology tests. Unlike other pharmaceutical pain relievers, the prevalence of fentanyl and other synthetic opioids does not appear to be tied to supply of prescriptions diverted to the illicit market, nor from people using opiates and craving a stronger high.
The most reliable evidence showing the presence of fentanyl laced into the illicit heroin supply comes from reports of seizures by the U.S. Drug Enforcement Administration. In the mid-2000s, the discovery of a single clandestine laboratory operating in Toluca, Mexico, between 2005-2007, reportedly manufactured enough fentanyl that seized samples of heroin during this time contained 20-25% fentanyl. The fentanyl laced from this Mexican lab was believed to be tied to over a thousand confirmed overdose deaths. Following a 2007 raid that shut the operation down, heroin seizures saw a drop in amount of laced-fentanyl (0-6%).
More recently, in April 2016, the New York City Department of Health and Mental Hygiene issued an advisory that among the 886 unintentional drug overdoses in 2015, 15% involved fentanyl, compared with fewer than 3% in all of the previous decade. In addition, in 2016, Illinois became the first state to declare two drug overdoses involving what may be a synthetic opioid known only as W-18, though authorities in Canada have discovered W-18 in seizures of counterfeit “fentanyl” tablets since 2015, and carfentanil has been confirmed in at least one case involving seizure of illicit blotter paper.
For the most up to date resource on monitoring of synthetic opioid and other NPS related news, the University of Maryland coordinates the National Drug Early Warning System, for communication between health officials, law enforcement, media, and disseminating information to the general public.