1)Aloud means to say something audibly so people can clearly hear you. It contrasts a whisper that cannot be heard.
2)Out loud means to say something loud enough to be heard. It is perhaps the preferred word choice to indicate a sudden outburst.
It comes with experience, they may be interchangeable, but sometimes they may not. It has to be dealt in a "case by case" basis.
But there are cases in which "out loud" can be understood as per definition #1):
Read the poem out loud. (loud enough to be heard) #1 suggests something voluntary. When referring to spoken voice, it´s the opposite of softly.
We'd already dealt with neighbors who were loud. (audibly/rudely/ringingly) #2, but then with the "outburst" quality.
She read it aloud : it asked for the pleasure of their company at luncheon. (she read it in a voiced way, not exactly shrilly - it´s the opposite of reading silently). So, falls into category #1.