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Maybe check the timing cover to see if there is a notch on the pulley and a mark on the cover. If so, just make it so they a somewhat out of alignment.
You could also pull an injector and stick a wire in the hole to feel the piston moving upward, or you could drop the pushrods in ( without the rockers in place ) and watch for them to move as you rotate the motor to determine where pistons are at, but to tell you the truth, the chances of you installing them with the motor exactly at TDC is slim to none. Even if it was at TDC you still shouldn't have a problem. You could install them and tighten the rocker arm nuts part way, until you feel contact with the valve tip and pushrod, go another 1/2 turn, then wait 15 mins. give another 1/2 turn, wait 15 mins more and tighten them down completely, then give another 15 mins or more before cranking it.
Turn the motor 1 revolution and let sit another 15 mins.
Even doing all that is probably being overly cautious and in excess of what the TM says, but should guarantee success regardless of piston position. I've installed plenty and never checked for TDC. You just don't want to dead crank or try to start it immediately after installing them, in case the lifters are still fully expanded.
