Hm. Your power supply should have a label somewhere on it. Depending on the model, it looks like the bracket holding the power supply should swivel out nicely allowing you to view the other sides. Hopefully, there will be a tag there.
Alternatively, and this is a pretty rough way to do it, if you can find your Dell Service Tag, you could enter that into
Dell's Warranty & Contracts site to look up the specs it was built with (although digging through the part numbers can be confusing). After entering your service tag, there should be a "Quick Links" section on the right of the page that has a link named "View product specs". Click "Expand All" and look through the list for a line that references wattage. I tested this with an example Aurora R5 tag and found the line "LBL,REG,460W,EPA,EMFP,R5" which references a 460-watt power supply. Like I say, this is a pretty rough way to do it.
Failing all that, the available PCI-e connectors on a power supply are only supposed to be there if they can provide the amount of power defined by the standard. 6-pin connectors can supply 75 watts, 8-pin connectors can supply 150 watts. As long as the power supply has those plugs available (e.g. you're
not using something like
a SATA to PCI-e 8-pin adapter or
6-pin to 8-pin adapter, etc) and you're not drastically increasing power limits/overclocking/overvolting your new GPU, it
should be fine. If you need to use adapters, you'll need to start paying close attention to your power supply's amperage rating to make sure you're not overworking it.