Distance from land and bilateral contracts.
In the North Sea the jurisdictions of Noway, Denmark, Germany, the Netherlands, Belgium, England, Scotland create almost a full circle around, leaving only a gap in the Iceland-bound area. There's a spec of Denmark that way, but that's typically not seen as in the North Sea. Where not otherwise possible, the EEZ maps for the area use the 12 Miles zones around any islands, but everywhere else extending to pretty much the mid of the North Sea. It is the general agreement that, where any zones would overlap, the exact middle is taken as the edge between them - or things are taken to court or politics.
Germany and the Netherlands did take the political route from 2012 to 2014, when they tried figuring out the Dollart Bay border between the two:
They established a special zone, in which the rules for construction and such are to be done separately from the actual borders based on some kind of demarcation line. But they only solved who is to be responsible for managing that stuff, not who is the actual owner of either side.
On the court route, in 1999, the Hague Permanent Court of Arbitrations decided that islands between Yemen and Eritrea are not to be taken into account for the EEZ and the exact middle between the mainland is to be taken. 2009 saw the UN ICJ rule that islands do not extend EEZ in the case over Snake Island and any island only projects the minimum 12-mile zone.
However, as we know from Greece, Cyprus, and Turkey: what constitutes an island and what mainland is very much up to discussion, especially in the case of Cyprus which might or might not be an island.