When a liqueur spoils due to oxidation, it does what you would expect a fruit to do. It turns brown, the sugars break down, and many suspended solids stick together and form a clump at the bottom. Often, the colors and flavors will separate considerably, leaving you with a dark, cloudy bottom half and a sickly yellow top half. It will never hurt to open up the bottle and taste it, but don't expect much. Some fruits rich in pectin may also gel.
Sugars often break down into glycerin, making the liqueur thicker anyway.
And the pectin inside may form isolated clumps of odd, alcoholic jelly floating in your bottle. It happens to my nectarine after two or three years, but I've never had the guts to taste the jelly. But a general rule of thumb is that when your liqueurs start to fall apart or change state, its time to toss them and start fresh. Corking As I've said elsewhere , there are plenty of good lessons to be learned from wine-making. But one I didn't mention is corking.