What is the best way to add mod support for non-API mods?

[tlr38usd]

Title says it. I want to add mod support for some other popular mods, but I don't want them to have to add any code on their end.  How would I check if their mod exists then reference the mods items. I have tried to use methods where my code searches for the mod jar and if it exists then it refers to the items by the ids, but that is not always the most reliable method and will differ from person to person.

[ObsequiousNewt]

Title says it. I want to add mod support for some other popular mods, but I don't want them to have to add any code on their end.  How would I check if their mod exists then reference the mods items. I have tried to use methods where my code searches for the mod jar and if it exists then it refers to the items by the ids, but that is not always the most reliable method and will differ from person to person.

If they're closed-source, you can't. If they're open-source, use reflection. What are the mods?

[tlr38usd]

Some of the mod authors I have contacted and given permission to add support is the authors of Metallurgy, Twilight Forest, Thaumcraft, and Tinkers Construct along with a few others.