Without drinking it, obviously.
This is continuing in the 'news you can use' category, among the trivialities that have been occupying my life of late while the events of the world pass me by.
I used to go with the smallest cup size offered by the cafe. There's a tendency among bad coffee shops to serve you up enormous bathtubs full of bilge water. Of course, to get a larger cup of coffee, they simply run the water through the same set of grounds until it turns into a burnt mess. The places that offer you a small sized coffee are more likely to know what they're doing.
But this was superseded by a tip from AL - the number of milk jugs on display. Good places will never heat their milk more than once. As a result, they tend to have a lot of small milk jugs around. If you see that, it's very likely somewhere that knows what they're doing. On the other hand, I've never had a good coffee from a place that had a single giant milk jug that kept being reheated.
If the place is failing the above signals and you still need a coffee, at a minimum order the smallest size you can.
(For the previous best signal, see here)