A mantel will help deflect the heat, depending on how far out it sticks, how high it is, and how far out the TV will be.
Also feel the spot where the TV would be mounted. If it is too hot to touch, then it will lilkely be too hot for a TV.
If you really want to be sure then you can get yourself a thermometer, run the fireplace for a while, and check the temperature at different spots where the TV will be (make sure you know how far out it will be). Take the highest temperature you get and compare this to the manual for the TV. There is usually an environmental operating temperature listed with most TVs. When comparing the two, keep in mind that the TV itself will raise the temperature as well with the heat that it generates, so make sure you have a bit of play. Measuring the temperatures is where the differences between gas and wood will really come into play. Gas fireplaces are pretty consistant with their output temps, varying only with flame height and external temperature. Wood fireplaces can vary their temps quite a bit. If you have a wood fireplace......safely load 'er up to get some good heat going.
Oh ya, and make sure you don't hold the temperature reading end of the thermometer!