Are all effects from spells considered magical? For instance, Life Drain from a specter forces a save on a hit. If the target of the hit has magic resistance does the save from that hit proc advantage on the save?

If you want a guide for determining magical effects, see the Sage Advice Compendium.

Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.

The Specter's Life Drain is a melee spell attack so it counts as magical, which would give you advantage on the save if you have magic resistance.