For some delays, the Eurostar Live Service Information page will list the delay for the service, and estimated revised arrival times. However, that doesn't always get updated as often as you might like...

For Eurostars still in the UK, it's possible to stalk them with quite some accuracy on RealTimeTrains.co.uk. If you search for departures from St Pancras International, find your Eurostar, then pick More details, you can get a display like this one for a train in September 2015 showing delays at 20-odd monitoring points on the way to the tunnel, eg

For Eurostars in France, you can't get that level of timing information WRT actual vs planned, at least as far as I'm aware. What you can do is follow the train's realtime position on the SNCF Geo-Localisation interface (zoom in on northern France), eg

That will at least give you a rough guide of delays, eg if your train is supposed to be in Brussels in 5 minutes and is showing as somewhere near Calais, you'll know it's quite late...

I'm not aware of any similar realtime train-stalking systems for Belgium, nor for in the Channel Tunnel itself. For those situations, you can try asking the staff in the Buffet (coaches 6 and 13), who can look things up if they're not too busy.

For some delays, the Eurostar Live Service Information page will list the delay for the service, and estimated revised arrival times. However, that doesn't always get updated as often as you might like...

For Eurostars still in the UK, it's possible to stalk them with quite some accuracy on RealTimeTrains.co.uk. If you search for departures from St Pancras International, find your Eurostar, then pick More details, you can get a display like this one for a train in September 2015 showing delays at 20-odd monitoring points on the way to the tunnel, eg

For Eurostars in France, you can't get that level of timing information WRT actual vs planned, at least as far as I'm aware. What you can do is follow the train's realtime position on the SNCF Geo-Localisation interface (zoom in on northern France), eg

That will at least give you a rough guide of delays, eg if your train is supposed to be in Brussels in 5 minutes and is showing as somewhere near Calais, you'll know it's quite late...

I'm not aware of any similar realtime train-stalking systems for Belgium, nor for in the Channel Tunnel itself. For those situations, you can try asking the staff in the Buffet (coaches 6 and 13), who can look things up if they're not too busy.