Dissipating 1W on a TO-220 without heatsink?
This is simple: do the math.  Look at the datasheet.  There should be a thermal resistance spec that tells you how many degC difference there will be between the die and ambient air per Watt.  Then add that to your worst case ambient temperature and compare to the maximum allowed die temperature.

For most transistors and ICs, a TO-220 case will get hot at 1W, but generally stay within operating range.  At 1/2 W I wouldn't worry about it.  At 1W I'd check the datasheet and do the calculation but it will probably be OK.

One wrinkle:  The datasheet may only tell you die to case thermal resistance.  You then have to add the thermal resistance from the case to ambient, which will be much higher.  Fortunately that's mostly a function of the TO-220 case, not the transistor, so you should be able to find a generic figure for that.  Good datasheets give you both thermal resistance figures.

Added:

I hadn't followed the datasheet link earlier, but now I see that everything you need is well specified in there.  The thermal resistance from die to ambient is 62.5 C/W, and the maximum die operating temperature is 175C.  You said your ambient temperature is 25C.  Adding the rise from there to the die at 1W yields 88C.  That's 87C below the maximum operating temperature, so the answer is very clearly YES, your transistor will be fine at 1W in 25C free air.