I am planning on planting vegetables and the old bed has decayed mulch which looks reddish and grounded. Do I need to dig out a few inches of this and replace it with soil? Is there a way to find out if this was organic or one of the treated mulches? This mulch was laid by previous owners so there's no way of knowing.
That's a real trick question. Even here people are sometimes advised to use pesticides for little diseases or pests that wouldn't kill the plants... And amateurs often use too much of the thing.
So it's a bet on your health to plant vegetables right away in this unknown soil. And if you remove 10 inches of it, whatis beneath can still be polluted and accessible by your plants roots.
From your picture, the mulch look natural to me however. You'd better have samples of your soil tested before eating your crop.
Interesting question. Going by probabilities alone, unless you can establish that the previous owners were running some kind of garage on the property, parked leaky vehicles there or other industrial use the chances are good that you are safe. Talk to your neighbours, invite them over for tea, chat about what the previous owners did on the land.
Next comes the mulch. There is a good chance that the mulch is fine. The only time it might not be is if the constituent dyed wood came from a source where wood preservatives where used. Normally the mulch people are professionals and will stand by their product. Take a trip to the local hardware stores and see what they are selling for red mulch, which is where the previous owners likely got the bags; read the labels, talk to the salespeople. Look for empty bags in the garage. Talk to the neighbours. Most of the redness comes from iron, and many soils are deficient in iron. The soil might or might not be low in nitrogen as a result of the mulch decomposition. Easily corrected.
Now what to do. Ideally we will get plants to do the work for us; first to send their roots down and shout out "I found some copper arsenate! Watch out!" or more likely "Nothing to worry about here." Second, we want them to take up the contaminants if any so we can remove the plants and take the contaminants away. In your reading look for the key term "phytotoxicity".
Plant a bunch of annual crops and see what does well and what does not. Have a little nibble here and there. Watch for leaf abnormalities. Avoid eating root crops which will have been in direct soil contact; focus on beans and leaves, where the soil moisture will have been filtered through plant cells.
For ease of mind, soil test.