For this question i can give you a simple straight answer , the answer is YOU CAN’T!!
We look exactly the same. Color, facial structure, Language, culture, tradition ( especially With Northern Ethiopia People). What you should understand is Eritrea was once part of Ethiopia and they used to be called Ethiopian. I’m an Ethiopian and i can’t tell the difference between Eritrean and an Ethiopian.
There are many different ethnic groups in Ethiopia, many with different “looks”. The same is true for Eritrea. However, one difference I did notice between Ethiopians and Eritreans is that Eritreans seem to be taller, on average, than most Ethiopians. This is a generalization as the people of Eritrea and Ethiopia are connected with many families having history in both countries.
If you can listen to them talking, there is a slight difference in their languages- I mean “Tigrigna”. There are people in the north-Tigrians they speak “Tigrigna”- their local language, the Eritreans also speak it. I can’t speak the language, but people say the difference between the Eritrean Tigrigna and the Ethiopian Tigrigna is there are many words from Amharic (Ethiopian National language) in the Eritrean Tigrigna. So even I ,an Ethiopian, cannot distiguish between a Tigray woman and an Eritrean one and most people would say so.