Is a glacier advancing or retreating? How to tell?
The easiest way is to look a the glacier margins. If the ice is in contact with vegetation or rock covered in lichens or moss, it means it is most likely advancing. If you see a band of life-less rock in between the ice and the first plants/lichens/moss, it means it is retreating.

Some examples I've seen myself:

This glacier in Geenland is advancing, you can see the ice and vegetation in contact

Same glacier, you can see how it is even pushing vegetation, proving it is advancing


However, advancing glaciers are the exception. In most cases they are reatreating and what you see is a band of life-less rock surrounding the ice like this:

Retreating Amalia glacier in Patagonia


The advancing or retreating state of a glacier can have different time-scales. For example a glacier can be retreating on a decade scale, but it might advance every winter, but the recession on summer is large enough to produce a net retreat every year. Small glaciers can also react very quick to weather variations, therefore even if one is receding at a century scale, it can advance one year due to exceptional snow accumulation over few years. So you have to interpret this observations with care, and consider what's the timescale at which the plants/moss/lichens can colonize the rock in the area, which in temperate and low elevations will be between a few years and a decade.
Moraines, trim lines and other features can tell you about the advancing/receding history of a glacier at longer timescales.