Indicators of a approaching warm front are: high cirrus clouds followed by a lowering base of Cirri-, Alto-and Nimbostratus clouds. Flying into a warm front you may notice lower stratiform clouds, ice pellets (depends on freezing level), freezing rain and rain.
http://www.experimentalaircraft.info/wx/weather-info.php
Ben Schott, NWS. Cirrus clouds are wispy, feathery, and composed entirely of ice crystals. They often are the first sign of an approaching warm front or upper-level jet streak. Unlike cirrus, cirrostratus clouds form more of a widespread, veil-like layer (similar to what stratus clouds do in low levels).
http://www.weather.gov/lmk/cloud_classification
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front.
https://en.wikipedia.org/wiki/Weather_front
In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is warmer than the cold air ahead of the warm front and rides over the colder air mass while lifting the warm air.
https://en.wikipedia.org/wiki/Weather_front
Southland, Ky-Stratocumulus Top of a building-NImbostratus. Stratocumulus clouds are hybrids of layered stratus and cellular cumulus, i.e., individual cloud elements, characteristic of cumulo type clouds, clumped together in a continuous distribution, characteristic of strato type clouds. Stratocumulus also can be thought of as a layer of cloud clumps with thick and thin areas.
http://www.weather.gov/lmk/cloud_classification
An air mass moving in with a higher temperature than the air mass it is displacing is called a warm front, see image to the right. Warm fronts move slower than cold fronts and as warm air is less dense than cold air the front tends to slope over the cold air many miles (up to 500) ahead of the surface front.
http://www.experimentalaircraft.info/wx/weather-info.php
If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
https://en.wikipedia.org/wiki/Weather_front
Altostratus clouds are strato type clouds (see below) that possess a flat and uniform type texture in the mid levels. They frequently indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus resulting in rain or snow.
http://www.weather.gov/lmk/cloud_classification
With moist and stable air masses you will find an extensive layer of clouds well ahead of the surface front. Precipitation will be zero if the air is dry and stable. As the front moves through, the wind veers, temperature rises (warm front) and the barometric pressure slows or stops dropping. When the front has passed stratus clouds and or fog can remain with poor visibility. Its the warm sector between warm front and following cold front. On the contrary, air masses with lower temperature overtaking a warmer air mass are known as cold fronts. See figure on the right.
http://www.experimentalaircraft.info/wx/weather-info.php
Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions.
https://en.wikipedia.org/wiki/Weather_front
which clouds are characteristic of a warm front?
High cirrus clouds followed by a lowering base of Cirri Alto-and Nimbostratus clouds are characteristic of a warm front. High cirrus clouds followed by a lowering base of Cirri-, Alto-and Nimbostratus clouds are characteristic of a warm front.