This is something you can certainly find out. Confrontation is the main one as simply put if you know your partner that well you will be able to tell if they are telling you the truth or not. The other option is of course to follow them around to see where they are going and is of course less trusting.
With advancements of technology you can now actually purchase a vehicle tracking system to actually put onto their car to see where they are going from the comfort of your own home. The real question to ask how trusting is this of your partner and just how legal is it? When installling the device the only legal restrictions you have are that you cannot break into the car or damage it in any way at all. Of course this also means changing the way the car is driven as this as you can in fact be charged for this.
You have to be very sure of your partner being up to something as if you end up being wrong on this and you get found out your marriage/relationship is pretty much over or at the very least has lost its trust. However it is not a good feeling to be paranoid and something like this could actually reek benefit if it turns out that your partner is doing nothing more than staying late at the office.
If of course you suspect that your partner is doing something at the office, then of course there is no way to tell if they are working or not unless you purchase something like one of those voice recorders and put it into the office. There are loads of other spy gadgets that you could also look into to make sure that nothing is going on with your partner or another woman/man.
Just be careful that the whole event doesn’t turn onto you and you find your partner is totally innocent as it will be you who is seen as the untrustworthy one in the relationship.