Here's one way:
Put a cooking rack right at the edge of the fireplace. Try to put meat on it.
If you can add meat, then it is too close: demolish it (you reclaim the meat as well as the wood, so you aren't wasting resources trying)
If you cannot add meat, but it looks close enough visually, that is what we are after.
Put a new temporary fire under it, add meat, and remove the fire. (maybe wait till the meat looks cooked?)
Result: meat on a rack, "close" to the fire, but not actually cooking