I used the following heuristics (some parts added afterwards):
1. Helvetica has level edges, Arial is angled (as ef4 said). Particularly important were "t", "e" and "a", "S", "G", C".
2a. For capital letters, if there is an "R", the Helvetica one is curved in the bottom right part while Arial uses a straight line.
2b. For a capital "Y", the Arial one has the same length in all directions while the Helvetica one is shorter at the bottom. (Alexx indicated a difference).
2c. The jags/gaps in the capital "M" extend further to the top for Arial. This can be used to figure out MATTEL.
3. Otherwise, the one that looks fatter is Helvetica.
1. Helvetica has level edges, Arial is angled (as ef4 said). Particularly important were "t", "e" and "a", "S", "G", C".
2a. For capital letters, if there is an "R", the Helvetica one is curved in the bottom right part while Arial uses a straight line.
2b. For a capital "Y", the Arial one has the same length in all directions while the Helvetica one is shorter at the bottom. (Alexx indicated a difference).
2c. The jags/gaps in the capital "M" extend further to the top for Arial. This can be used to figure out MATTEL.
3. Otherwise, the one that looks fatter is Helvetica.