Depending on the cellular specificity of your Cre expression, you could do a immunofluorescence on different tissues staining for your specific cell type and check if the TdTomato expression is restricted to those cells.
Typically you would use a Cre-reporter line (eg Rose-lox-stop-lox-lacZ) and characterize Cre activity, confirming activity in target tissue and note off target activity. Also check the Mouse Genome Informatics website (MGI at the Jackson Laboratory). There’s Cre mouse resource that compiles data on available Cre lines.