Poplar.
Easy enough to mistake, they weigh very nearly the same, poplar is slightly heavier.
Hardness is nearly the same, alder is a little bit harder at 590 on the Janka hardness scale and poplar is 540.
The color of poplar can lean to toward the grey and greenish while alder can get a clear coat or a trans finish or a sunburst and look better.
Poplar is generally less expensive than alder but alder was chosen by Fender all those years ago because it is cheap and abundant in the west in the USA.
According to Warmoth, alder and poplar sound the same, ...if you believe in the tonewood thing on electric basses.
Please take some pics and post them, everybody likes Bass Porn.