Hope i'm doing this right.....

i'm trying to wire up a dimmer switch (not too difficult I guess) but my
problem is this..... Both the wires on the existing light switch are red so
how can I tell which one is 'live'?? The connections on the current switch
are marked L1 and Com (i think), but this doesn't help me???

I'm sure it's a simple answer - but i'm not electrically minded.

Your assistance is greatly appreciated.

Many thanks.