Hello... unless it's a Old Dell... depends how old it is and who made it (Old Walmart computers typical) If you are looking to replace a OLDer model with a NEW one?... A easy way to tell is just look at the PS connector... here is a ATX STD wire color coded mid-page... do the wire Colors and their positions match?
https://en.wikipedia.org/wiki/ATX
Typically I have found all newer PS's and MB's from Dell and the BIG NAME MB companies to be ATX connector compatible.