Empty gun powder containers and especially disassembled but non-detonated commercial firecrackers is another potential indicator of the manufacture of improvised explosive devices because the latter provide a means of securing sufficient quantities of gun powder to create a bomb by persons (i.e. juveniles) who cannot ordinarily purchase gun powder, as well as by individuals intending to avoid creating a record of having purchased gun powder. Likewise, the presence of hobby fuses, particularly without other contextual materials suggesting their legitimate use (i.e. model rocketry materials) are another potential indicator of improvised bomb-making activities. Hot glue guns (also outside of contextually appropriate circumstances) are another potential indicator. Recognizing and Avoiding Detonating Possible Tennis Ball Bombs

Tennis balls and racquetballs with holes or slashes are obvious warning signs of explosive devices improvised from those items. Similarly, tennis balls and racquetballs covered in electrical tape or duct tape are...

In light of their potential danger, destructive power, and mechanism of detonation, vehicle operators should never drive over balls. Any discovery of a potential tennis ball bomb should trigger appropriate notification of police authorities by members of the general public and standard operating procedures for securing crime scenes first and then notification of emergency services and/or bomb squads and federal ATF hotlines by members of police services.
Sources:

(1) http://www.instructables.com/id/EHX8XOXR7ZEP4HD614/

(2) http://www.wikihow.com/Make-a-Match-Head-Tennis-Ball-Bomb

(3) http://www.theherald.co.za/herald/2003/11/14/news/n12_14112003.htm

(4) http://www.highbeam.com/doc/1P2-6507864.html

(5) http://www.highbeam.com/doc/1P2-6507864.html

(6) http://seattlepi.nwsource.com/archives/1996/9609110027.asp