Capacitor vents are intentional weak points built into larger capacitors to prevent the capacitors from causing serious personal injury when exploding. While these vents are present on all newer capacitors of a certain size, they may not be present on older capacitors.
Capacitor vents are different shapes for different brands of capacitors. You may be able to tell the brand of capacitor by looking at the shape of the vent. Keep in mind that the vent shape is not a clear indicator of a certain brand or capacitor; cheap capacitors will often steal vent shapes from more well-known capacitor manufacturers and some manufacturers will use multiple vent shapes depending on the age and/or series of the capacitor.
Cross-Vent/X-Vent (Nichicon, Panasonic, Wurth)
The X-vent shape is used on Nichicon and Wurth capacitors, in addition to some Panasonic capacitor series.
The X-vent is also used on the following capacitor brands: Chang, Chengx, Econd, GL, Hycndz, J.d(Gree), KSC, Ltec, OST, Samson, Samxon, SC, Senju, Taicon, TK(Toshin Kogyo), Xficap, YEC
The vent on Elna capacitors are a specific shape and don't seem to be present on any other brands of capacitors.
The K-vent shape is used on Rubycon capacitors.
The K-vent is also used on the following capacitor brands: TK(Toshin Kogyo)
The T-vent shape is used on Panasonic capacitors.
The T-vent is also used on the following capacitor brands: Hermei
Y-Vent (Nippon/United Chemi-Con)
The Y-vent shape is used on Nippon Chemi-Con and United Chemi-Con capacitors
The Y-vent shape is also used on the following capacitor brands: Aishi, Fuhjyyu, Fujicon, Jamicon, Ltec, Sinecon, Telecon, United, Ymin(Yongming Capacitor)