Was it largely a jurisdictional divide? Or were there actual border checkpoints that regulated the movement of people? Was it something else, perhaps something looser than that, like with communities close to ‘the border’ having different loyalties that didn’t necessarily align with which side they were on?
It slightly depends on what you call the "height" of the Danelaw, but much of the border was heavily militarised, although not necessarily in the form that we might recognise today. The extent to which the line agreed in Alfred's treaty with Guđrum represented a hard border is quite hard to say. Certainly by the end of the 9th Century, Mercian rulers are granting land to their ministers that technically violates the treaty.
Much of the Danelaw border was with Mercia rather than with Wessex, and this area was indeed heavily militarised by Æthelred and Æthelflæd in the late 9th and early 10th centuries, albeit on a pattern copied successfully from Wessex. The prime avenues of approach into the Mercian heartlands came from the direction of Derby, where there was a major Danish stronghold, through the Aire Gap via what is now Manchester (South between the North Sea and the Peak District) and maritime raids up from the Wirral or from the North Wales coastline. Central to the Mercian defence of these borders was a series of burh fortifications at Chester, Eddisbury, Stafford, Tamworth, Runcorn, Bridgnorth, Chirbury, Hereford, Warwick and Worcester. Like their West Saxon counterparts, Mercian burhs were garrison sites for semi-permanent detachments of the fyrd militia who could deny vital transport routes to the Danes and use those routes to respond quickly and combine forces against any aggressors. For this system to function, it required a series of watchtowers and signal fires along the frontier and strategic routes which could quickly raise the alarm in the case of raids or invasion, warning local populations to seek shelted and summoning garrisons and helping to triangulate them on target.
From Derby, for example, a Danish force could either strike South along the River Trent or its accompanying Roman road in the direction of Lichfield and Tamworth, or West along the River Dove in the direction of Uttoxeter and the major settlements around Stoke-on-Trent. They would, however, be spotted by the tōt site at what is now Tutbury, which would in turn alert a Tothill near Uttoxter and another Tothill near Hollington, and so on until alerting the garrison forces at Stafford and Tamworth, and potentially as far as Chester and Eddisbury if necessary. While we wouldn't therefore see what we today might call a hard border - with fences, ditches etc. like we might see at Hadrian's Wall - it was nonetheless a militarised border where we would see defensive surveillance and signalling positions.
While much more can always be said on the topic, I hope my previous answer in: (SASQ) In 909 AD, a combined forces of West Saxon and Mercian raided the Danelaw in order to retrieve St. Oswald's relics. Did they view Danelaw as another kingdom or just Anglo-Saxon territory occupied by the enemies? might be interesting to OP.
It also depends on the perionization of "at the height of the Danelaw".
In short, while some kind of traffic control had been implemented at first, the authority of Wessex rulers had probably penetrated in so-called Danelaw area already before its military (re-?)conquest in course of the early 10th century.