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The actually a WARNING will be at runtime logged at the publisher side:

E. g. if trying to publish a std_msgs/Emtpy to topic /camera/image where rosrun image_view image_view is listening:

rostopic pub /camera/image std_msgs/Empty -r 1
[WARN] [WallTime: 1426844976.659990] Could not process inbound connection: topic types do not match: [sensor_msgs/Image] vs. [std_msgs/Empty]{'topic': '/camera/image', 'tcp_nodelay': '0', 'md5sum': '060021388200f6f0f447d0fcd9c64743', 'type': 'sensor_msgs/Image', 'callerid': '/image_view_1426844958567455361'}

On the subscriber side, however, no Error/Warning is logged:

rosrun image_view  image_view image:=/camera/image
[ INFO] [1426844958.672731010]: Using transport "raw"

So you have to look at the log msgs of the publisher.