|Okay, what is your mushroom/fungus growing on?|
|Leaves - live|
Your fungus is probably a pathogen, getting its nutrients from the live plant. Its ecological role is therefore that of a Parasite.
|Leaves - dead|
If your fungus is growing directly on dead leaves, it is probably getting its nutrients by breaking the dead leaf down. Its ecological role is therefore that of a Saprobe.
If your fungus is mushroom-shaped (it has a stem and a cap), it is probably a Saprobe on a dead stem, or a Parasite on a live stem. If it is blobby or oddly-shaped (like the Sebacina shown in the photo), it has probably formed a mycorrhizal Mutualism with a plant.
|Other mushrooms - weird!|
If your fungus is a moldy-looking or colorful coating on a mushroom, your fungus is a Parasite. If your fungus is mushroom-shaped and grows on the old, blackened remains of another mushroom (see photo), it is a Saprobe. that digests the older, rotten mushroom.
If your fungus is growing on something else, chances are that it's breaking down dead material and is therefore a Saprobe.