I'm familiar with the 3.0l M103 engine but I think the 2.6l M103 engine is similar.
The real way to check cam timing is by setting a dial gauge on the #1 intake tappet and checking for crank rotation of 11-12 degrees ATDC by 2mm of tappet travel.
To know this, you'd have to have a manual so I'll assume you don't have one. This is not an easy engine to figure out without a manual.
It's dangerous to turn the engine if the cam timing isn't right. I'm not sure what you mean by 'everything is back together,' but I would start with removing the cam before turning the crank to TDC if you have any reason to believe that the cam timing is way off.
If the cam is not on, turn the crank to TDC. Keep some tension on the chain so it doesn't bunch up where you can't see it. Install the cam with the pin or hole (I think an 87 will have a hole) in line with the mark at the top of the #1 cam bearing. If it has a pin, it'll be on the back side of the flange the sprocket bolts to. If it has a hole, it'll be on a ring just ahead of the #1 bearing surface.
Now the guessing begins. The sprocket fits in any of 3 ways to allow fine tuning of timing. Maybe you can tell by inspection which of the 3 trailing holes on the sprocket matched up with the pin on the front of the camshaft. If not, you won't be off by so much that you can't turn the engine.
I don't know if it's sufficient to turn the engine by hand with the tensioner in place to check cam timing. I mean you can turn the engine by hand, but I don't know if the tensioner will take enough of a set that the cam timing you observe is necessarily correct. Someone else will have to advise you on that. The point is that you can check cam timing after the tensioner takes a set and use the other sprocket positions to get the cam timing mark closer to where it should be.
Don't forget that these engines have a ratcheting tensioner. It has to go back on in pieces or it might put too much tension on the chain.
ps Are you changing your handle to HP?