Installing front piston and with the arrow facing forward the larger valve relief lines up with the smaller exhaust Valve. 87 electraglide

 

If stock pistons, then arrow on both pistons will be pointing to front of bike.

If aftermarket pistons, then need photo on the top and bottom side of the piston, and need you to add in direction of arrow on top of piston to the bottom of photo as well.
And to add, really easy to tell which way the piston goes in, since you have the valve relief cuts on the top, but more so the lugs on the piston that tell you which side will be sprocket side to start with.
https://www.hdforums.com/forum/evo/1...ntation-2.html

 

If it's after market they might use different markings.
The arrow might point towards the intake port for that brand.
Consult with the manufacturer.