You are fine to plant them if you have a sim that can you just can't leave them in inventory in case it's a death flower, as for telling what they are well Plasma is obvious as it glows, I've read Nutella mention that death flowers and money tree seeds have a 0 value so might give you a clue there, that just leaves the flame fruits and life fruits.
Also remember when harvesting to take the death flowers off any non immortal gardeners once they collect them, it was actually in my dynasty that a sim gave Grim a death seed, I was so shocked but thankfully allowed to carry on but it meant this new ruling came about.