how do I check if they are in Acoustic phase? and is this referring to the slope used on the x-over point?
acoustic phase is the phase of the wavelength when it reaches your ear(s). high frequency wavelengths are really too short to make this a concern, unless you drive with your head in a vise. just concentrate on relative intensity at this point.
The slope used only affects the phase at the crossover point. It determines whether you will have a peak or a dip at the crossover point. Acoustic phase has more to do with how far the drivers are from your ears. If you move a Speaker further/closer to you its acoustic phase changes in relation to its relative distance to you.
I turn off everything else except the tweets, lower the volume, drop the crossover to 1K and use a verified track to align everything up by ear. Raise the crossover back up, turn everything back on......