Other than a forward's plus-minus rating, what are some other indicators that he is an asset to his team for being a defensive forward? I imagine hits would be one. PIM seems like it would be a detriment.
The biggest statistical indicators of a defensive forward in the NHL are going to be some new advanced "fancy stats" that have been taking the NHL by storm recently. The two main ones for players are Corsi and Fenwick, which are actually very simple. Corsi is just a player's +/- for shot attempts, and Fenwick is just a player's +/- for shot attempts not counting blocked shots.
We can do a bunch of neat things with them, like finding someones Corsi%, which is their Corsi For / (Corsi For + Corsi Against); here, Corsi For is shot attempts that the player's team took when he was on the ice, and Corsi against is shot attempts made by the other team when he was on the ice. Another useful thing to look at is a player's Corsi when the game is "close", meaning that it's within 1 goal. Player usage changes when teams start to get large leads and it skews stats.
Corsi% is very useful at finding efficient players; however, it weighs offensive and defensive contributions the same.
One thing you can look at also is called Zone Starts. If a forward routinely gets sent onto the ice when the faceoff is in his own zone then perhaps his coach views him to be good defensively.
Lastly, Takeaways is also a good stat to view; whether or not a player is adept at taking the puck from the other team.
These can all be found on NHL.com's stat page if you go to "Players" and then under the "Report" drop down menu, select what category you're interested in.
I hope this helps!