There MUST first be dye installed and it must be installed for a long time usually, long enough for the freon charge from the last re-charge (w dye added) to leak out again and that can be as long as a season or two if it's a slow leak.
So, assuming that that's been done and it's low on freon once again, you can see the evap by removing the right knee bolster, then remove the blower resistor. You'll need a black light to then see the dye.
Any other way and one's just guessing.