For the most part, tube trains travel on the left. So, if you have a reference to the other platform, you can deduce in which direction the train will be travelling.
On a few of the older cut-and-cover lines, we have two tracks running down the middle with platforms on either side (i.e., you can look across the tracks and see the other platform). For example, this is true at Monument, Sloane Square, Fulham Broadway, Cannon Street and many other stations on the Circle/District lines. Here the trains travel on the left.
At smaller, newer stations where there is only one line, the platforms are usually in the middle of the two tracks. Sometimes there is a single "island" platform (e.g., South Kensington) but on the deeper lines the platforms are separated, in their own tunnels: either way, the principle still holds. Again the trains travel on the left, so you can use this to infer the front of the train.
At big stations where many lines intersect, it is harder to follow the principle; sometimes the trains switch sides or the platforms are optimized for connections between different lines. A close look at the actual track and platform arrangements illustrates these patterns as well as some of the notable exceptions:
- Trains run on the left
- Deep tube platforms are situated between tracks
- Sub-surface tube platforms are situated outside tracks
As a result, deep tube trains generally run to your right as you stand on the platform facing the tracks, while sub-surface trains run to your left.
- Platforms are sometimes rearranged (e.g. Platform 2 at St. Paul's is outside the tracks rather than between as is generally the case for deep tube; Platform 1 at Mansion House is between rather than outside as is generally the case for sub surface lines; At Piccadilly Circus, Oxford Circus, and Embankment, both Bakerloo platforms are outside rather than between the tracks)
- Rarely, trains run on the right (e.g., the Northern line at Bank and at London Bridge).
As a result, deep tube trains sometimes run to your left (Bakerloo at Piccadilly Circus, Oxford Circus, and Embankment; eastbound Central trains at St. Paul's) and sub-surface trains sometimes run to your right (westbound Circle and District lines at Mansion House; Northern line at Bank and London Bridge).
At the platform you may be able to spot the the clear ("go") signal lights (white for deep lines, green for all other lines) located at the driver's end of the platform. This is probably only going to be illuminated when the train is already there, of course, but you may be able to see the light regardless.
In the end, once you do a journey quite often you will automatically find yourself going to the most optimal part of the train and it becomes second nature. If you are only doing it once, it probably is not so important.