Mustluvcats:
That's what I did, too, including the non-season-friendly recolours of Maxis plants. I first tried to make seasons plants exactly one year ago but failed, so it has taken me quite some time to get the hang of it. And I'm still just tweaking existing meshes.
Mapat:
Apart from making a test lot with a test sim, planting all the trees you have, cheat the sim up to gold aspiration, buy a weather machine and change the season a few times to look at all seasons?
That's what I did.
Or you can open each tree and shrub in SimPE and look at the GMDC. It there is only one resource there then the tree will not change with the seasons. If you see more, probably ending in swx01_tslocator_gmdc or similar, then it should.
There is one exception to this, though: plants that are not actually trees but deco objects. Those will not change either, but they may be covered by snow in winter if the creator enabled it.
The same is true for shrubs about the GMDC and the deco plants. I am not sure about flowers, I haven't gotten that far yet.