Identify Transformation Used by Append Tool

[DETCOG_GIS]

In ArcMap, I regularly use the Append geoprocessing tool to append data from a dataset in one projection (in this case UTM 15 N, meters) into another dataset in a different projection (in this case WGS84).  When you append between projections, as expected, there are slight shifts in the data.  I'm fine with this as the shifts have always seemed to be consistent, meaning it appears to use a consistent transformation in the process.

However, I'm now in a situation, due to some additional datasets in other projections that I am having to edge match against and other projections editing is being done in, that I need to know what transformations are actually being applied by the append tool.

I know in the project tool, you can specify what transformation is used, so you could move data back and forth between coordinate systems in a predictable way.  However, I've not been able to figure out what transformation the append tool is using, to let me and other users work out this cross-projection edge matching issue.

And to note, I'm not talking about just the on-the-fly projecting done within ArcMap/ArcPro for display purposes.  I'm talking about changing coordinate systems of the actual datasets themselves.

If anyone has a lead on this, I'd appreciate!

[DrewFlater]

Geoprocessing uses a default/best transformation when on-the-fly reprojecting datasets between different datums. The only place we have the auto-selection of this transformation visible is from the Project tool. So I suggest you can use this workaround/

Lets call your Append input Data_A, and your append target Data_B. 

1. Open the Project (Data Management) tool, select Data_A as the Input Dataset.
2. If Data_B is in the map, for the Output Coordinate System parameter you just need to select Data_B from the layer choicelist. If it's not in the map, click the button to select a coordinate system, use the button to import a coordinate system, then in the dataset browser find and select Data_B. 
3. Back in the Project tool, the default Geographic Transformation between the two datums is populated. 

[DETCOG_GIS]

Thank you for the info! In this particular case, further info identified that the new datasets I was matching with were in different projection but the same datum (NAD83) as what I am already editing. So the skew introduced by projecting to WGS84 appears to be the same between the datasets, eliminating some of the need to know this for now. But this will likely greatly help in the future as other data sources get added to this project.