Yes you can do that to get general idea of TDC, but there is some crank travel at top without piston movement, you wouldn't even need to pull valve cover just use long small rod or screwdriver. But it would also help to confirm how close the balancer notch is. The notch at top of balancer is a major problem if it hasn't slipped in bonding, which is direction it would move if slipping as the load of alternator and /or A/C compressor would pull it that direction.
If notch ends up close to "T" when you check TDC thru plug hole of #1 cyl. that would indicated timing belt did jump at crank pulley, which is not unusual as
@OhioSpyderman indicated.