Remove the left side top rocker cover. If the intercooler is in the way remove it first.
When number one piston is at TDC on the compression stroke both valves will be closed and there should be right around .004" rocker arm elephant foot to valve stem clearance so you will be able to wiggle both rocker arms that tiny little bit.
You'll feel the .004" clearance and you may hear the faint clicking as you wiggle the rocker arms back and forth.

thanks

And to add to what JFairman described... If the #1 piston is at overlap (meaning #4 is at TDC) the rockers will be tight due to spring tension.

Its pretty easy to tell if the rocker is "loose" indicating TDC or "tight" indicating cam overlap. If #1 is tight, rotate crank 360 deg and it should be loose.

If the rocker on #1 is slack - TDC on firing - as above. When you go another 180 deg you are on the cam timing stroke and the rocker should have depressed about 1-2mm. So, easy to tell if you take the cover off. If side 3-6 is easier to get at - less crap to remove - take that inlet cover off. When crank at TDC mark, if #4I is depressed 1-2mm, then #1 has to be on TDC firing/comp.
Alan