How dry is the prime concern for me as the type matters very little once the sap/moisture/tannin are gone. I often use cedar once it is totally dry but the question of how long that takes is still an open question for me as conditions matter a whole bunch. What dries in a few years in Texas may take far longer in a climate where it is cool and humid. For large cedar, even in the local heat, it takes at least ten years to get safe and not bleed tannin to color the water. Along with tannin that color the water, aging removes the "toxins" that many fear in the softwoods like cedar and pine.
That leaves me to test before deciding. Since wood does dry from outer to inner, I look first at the outside and know that bark dries first and often peels or flakes off when dry. So is there bark still firmly attached? Odds are that the wood underneath is not dry and I don't bother to pick up that piece.
A second step is cutting an end off the larger part to get a look at the inside. Wood normally changes color as it dries so if I find a generally uniform color all the way through, I call it dry. Exceptions are around knots, etc. where it does dry slower but if there are not a large number of those places, the tannin content is also expected to be low.
What I don't want to see is a cut that shows an outer ring of one color and the center another which often indicates it is still "green" there. Some wood does have a center core that is different and it does take some judgement on what you are seeing.
It is my general idea that there are very, very few woods which are truly toxic but the sap is a totally different thing and I want to avoid it as it is just too much trouble on several points.
Dry cedar, cut and trimmed to add space for fish hiding when turned over. Second picture is the wood in use from my pre-planted days shows rainbow cichlids protecting fry in the little black gob under them. Grandpa visiting day, perhaps?