Comment: Spruce and fir look the same from a distance, and even up close to a lay person. Fir needles are more flexible, flattish, and attached to their twigs in two rows, one on each side; spruce needles are stiffer, square in cross-section, and spiral around their twigs, so the twig-with-needles is bushy and round like a big pipe cleaner instead of flattish like a big stir-stick.

Obviously if both are mixed in an area it’s even trickier, particularly if one of them is infrequent. You could examine several trees, find they were all fir or all spruce, and think the other absent in that case.