From the Icelandic Met Office, who are responsible for monitoring volcanic activity:
- The seismicity during the past month has been above average
- Geothermal activity has increased over the past months with clear signs of deepening cauldrons in several places around the caldera
- The surface deformation has exceeded the level it was at prior to the 2011 eruption
- Magmatic gases were measured in the geothermal emissions this summer.
"Multiple datasets now indicate that Grímsvötn volcano has reached a level of unrest, comparable to that observed prior to historic eruptions."
True, any volcano might or might not erupt; increased activity in Reykjanes now seems to be subsiding.These are all strong indicators though.