Solution: Fill a small spray bottle with vodka or better yet, grain alcohol and spray the nozzle in the end of the tap. Hold a bar towel around the end of the nozzle to prevent dripping.
Problem: Beer comes out foamy.
Solution: Pull the tap on quickly. This is the number one mistake patrons make at my home bar. If this doesn't work, perhaps the problem is larger. Be sure each tap line is at least 4-5 feet long (longer is preferred). Warm beer will foam more. Finally, serving pressure can normally be very low. I have four taps and my psi is always below 10 and typically set between 5-7. Inspect the liquid side poppet and replace or lube if necessary.
Problem: Keg lid won't seal.
Solution: Set your PSI to around 30 to "seat" the seal, then you can reduce it it whatever you like. Also be sure to lube the large o-ring with food grade keg lube. If you are still having trouble, try reversing the keg lid 180 degrees and see if that helps. Sometimes exchanging the lid with a different keg will do the trick. Finally, put a coin under one or both of lid arms to apply extra leverage to the seal. Spray some starsan around the o-ring or submerge in a tub to confirm it holds pressure.
Problem: How much beer is in my keg?
Solution: Weigh the keg. A gallon weighs 8.34lbs so that puts a full keg at approx 51lbs factoring in 8.5lbs for the weight of the keg. This works out nicely because there are 52 beers in a keg.
Another solution is to put a temp sensitive strip on the side of the keg which will show you the approx beer level. Moving a keg from cold to room temp will show a condensation line after a few minutes also showing the level. In-line flow meters are another great option albeit expensive.
Problem: CO2 regulator PSI rises slowly over a few days.
Solution: This usually means one or more of the kegs contain beer that is over carbonated--even if only slightly. Be sure to bleed off all the shared head space to be sure the set PSI is maintained. Periodic bleeding may also be necessary.
Problem: How can I bottle from the tap?
Solution: It's easier than you think! First, the beer should be as cold as possible and you should spray and empty each bottle with sanitizer to reduce foaming. Next, set the PSI at the regulator very low (around 2-3) and bleed all the CO2 from the keg to be used.
Alternatively, you can remove the gas side ball lock from the keg and bleed the CO2 until the hissing sound begins to diminish. At this point, there's usually enough CO2 to fill a 22oz or 2-12 oz bottle. It also helps to considerably reduce foaming.
If you have a way to blow a burst of CO2 into the bottle before filling, this will reduce oxygen contacting the beer. Finally, place the end of bottle over the tap and pull the tap handle on very quickly being careful not to overfill. A cobra/picnic tap will also work using the same method.