It depends on the change in vote in the ward, and what the vote normally is like.
If you've got formerly safe Tory and Labour wards losing huge amounts of votes and losing seats, while other parties don't pick those votes up, then that's a major signal, particularly if the vote has tanked in a certain polling zone that you know delivers a core vote.
It was clear in ours that the Tory councillor lost his seat because of the national mishandling of Brexit. His vote share collapsed by over 50 percent (which is extraordinary), as did the Labour vote. Lib Dems didn't pick those lost votes up and neither did the Greens. In other wards, lost Tory votes resulted in Labour winning a seat from the Tories despite a tank in the Labour vote as well.
It's not just as simple as overall numbers of seats lost nationally. Plus you've feedback from local associations who will tell you why your candidate lost, and they know from canvassing.
Besides, Ukip doesn't field candidates everywhere and the Libdems are seen as a safe place to park a local vote because they won't win a general. As an example, there is a hardcore brexit zone in the ward next to mine. Last election, the majority of votes were for Ukip. This time, no ukip candidate stood, so they all voted Libdem because they won't vote Tory or Labour due to their stance on brexit. This might not make sense at first glance, but voting is often about refusing support for a certain party as much as giving it.