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The Guardian Portrait from the Curse of Strahd adventure (p. 227) has the trait:
False Appearance. While the figure in the portrait remains motionless, the portrait is indistinguishable from a normal painting.
Assuming the Portrait casts a spell with somatic and/or verbal components - is the detection of the Guardian Portrait automatic by PCs or is this something for which they need a Perception check? How did the Curse of Strahd adventure define how these Guardian Portraits are discovered?
For starters, you might want to check out this question about noticing spellcasting, and/or this Sage Advice mentioned by @DavidCoffron in a comment.
Beyond that, and in case you desire a way to telegraph the fact that the portrait is animate, I will tell you what I did for my game, since my players actually encountered that creature.
About the portrait itself:
Chapter 4: Castle Ravenloft, Room K47, P. 69-70 "The portrait on the wall depicts Strahd von Zarovich before he became a vampire. Even in life, he was pale. The eyes of the portrait seem to watch and follow the characters as they explore the area. The picture frame is bolted to the wall and can't be removed without destroying it."
So from that, when combat was initiated, and the portrait began casting spells, I had simply made the picture portion animated, like a Harry Potter picture. The image of Strahd was actually gesturing and speaking.
The adventure book, however, does not present a way. It's completely up to the DM, and I found that being straightforward and to-the-point was effective for me and my group: "The thing is casting at you, dangit."
Ultimately, as a DM, it's up to you to decide if and how the players discover it.
Detect Magic reveals magic around the user as follows:
For the duration, you sense the presence of magic within 30 feet of you. If you sense magic in this way, you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic, if any.
So, any Character that uses Detect Magic would sense that the Portrait (an object) bore Magic, and might reasonably assume that the Portrait was a Construct or, at the very least, a trap of some sorts.
Since the Portrait has Antimagic Susceptibility, it has an ongoing Magical Effect which could also be Detected through Detect Magic, as, otherwise, the Portrait would not be able to be neutralized through Antimagic Field, which also only affects objects when they have current or ongoing Magical Effects.
Assuming the party can see (somatic) or hear (verbal) then casting the spell reveals the portrait for what it is.
Notice that the guardian portrait specifically has the Innate Spellcasting feature for the spells it can cast. The feature specifically calls out the removal of material components, but the book Xanathar's Guide to Everything provides a potential interpretation for a Dungeon Master. On p. 85 under the subheading "Perceiving a Caster at Work":
If the need for a spell’s components has been removed by a special ability, such as the sorcerer’s Subtle Spell feature or the Innate Spellcasting trait possessed by many creatures, the casting of the spell is imperceptible. If an imperceptible casting produces a perceptible effect, it’s normally impossible to determine who cast the spell in the absence of other evidence.
A DM could reasonably interpret this as to affect verbal and somatic components if they so wished, since as written this only affects the portrait's casting of telekinesis. This would certainly be a case of RAI, however.
