How to determine how much beer is in your lines:

fill your lines with water
pull off the water from a beer tap into an empty vessel
for each tap, measure and record how much liquid was pulled off
make this information available to your staff so that beer in the lines can be effectively traded-out.
keep clean lines

To produce perfect pints, you must prevent yeast growth in your beer lines.

The effects of yeast build-up may include:

Line cleaning should be:

regular - at least every seven days
thorough - line cleaning is about hygiene, not just getting rid of any visible spots of yeast
done with a high quality cleaning solution.
Remember: don't leave cleaning solution or water in your lines to soak overnight. This could damage your lines and make your beer taste like soap.

How much beer is there in my lines?