There's a few things that I personally do to avoid getting shieldgrabbed.
1.Fair to Dtilt. First, the most important part is to space your fair. You want hit just with the tip of your sword, although you probably already know that, and then L cancel into a dtilt. This will make sure you don't get shieldgrabbed, as long as the spacing on your fair is good.
2.Fair to grab. If the fair hits with the tip, then it should cause enough shield stun for you to L cancel and run in for a grab.
3.Nair to Spotdodge. I find it easy to Nair someone's shield and then spotdodge right after. It's fast, so you shouldn't get shieldgrabbed.
I would say Nair or Fair to jab, but it's risky. His jab is slow, and ir gets shieldgrabbed a lot also. I wouldn't recommend it.
But ya, the most important part is making sure you hit their shield with the tip of your sword. We all know how long Marth's sword is, so you'll be safe.