In Albanian-majority areas, it's safe to say: Serbia has no control whatsoever.
In the Serbian enclaves in Kosovo, particularly in Northern Mitrovica, which held its own referendum in 2012, unrecognized by Kosovo or the EU, ... any control Serbia might have is rather unofficial, i.e. at the level of influence. But this is fairly extensive:
In northern Kosovo, streets are lined with Serbian flags, the Serbian dinar is the currency, Serbian government companies provide many utilities, and the language spoken on the streets is generally Serbian. Cars cruise around with no number plates; those with official Kosovo plates remove them on entering the north. By contrast, South Mitrovica on the other side of the Ibar river is almost entirely Albanian, and administered separately.
On the other hand, Kosovo controls well its border, even in this area:
Meanwhile, the Kosovo customs checkpoint is also working, and that has real-life consequences for Serbs trying to make ends meet in northern Mitrovica.
One of them is Blagitse Inackovic, a 44-year-old former office worker laid off from Mitrovica's Trepca mining company in 1999 -- along with 23,000 others on both sides of the Ibar River -- when the firm halted most operations.
The unemployment benefit in dinars that Inackovic receives from Belgrade each month is not enough to feed her two children and pay for their college educations. So Inackovic and her husband, also a laid-off Trepca worker, run a fruit and vegetable stand in northern Mitrovica where they accept both dinars and euros, the official currency used in the south.
Inackovic's worries multiply on market day, the Saturday after the latest incident at Rudare.
Her son Zoran is stopped by customs officials as he tries to bring a fresh supply of produce from central Serbia through the crossing. By the time he arrives with the delivery, market day is over. Inackovic explains that by Monday, when she opens her small business again, the perishables will no longer be fresh and will be difficult to sell.
Occasionally Serbia tries to assert more control/influence, but mostly without success particularly when it comes to border-crossings. E.g.,
So it's an ongoing power struggle as to the level of de facto control Serbia has in the Serbian enclaves in Kosovo.