Stephen Brunet did in his article "Female and Dwarf Gladiators"
Museion XLVIII - Series III, Vol 4, 2004 a classification of evidence of female gladiators as follows:
1.
Historical References
Cassius Dio (61.17.3-4) and
Tacitus (Ann. 15.32.3) both report that Nero induced women of highest rank to appear in the arena, but not only noble women but also men of the senatorial and equestrian rank. It is not clear if they refer to the games in 59 upon Agrippina’s death or to the year 63. But it seems to have been only one occasion where Nero made upper class women become gladiators.
Martial Sp.6 and
6B and
Cassius Dio 66.25.2 mention female venatores at the inauguration games of the Colosseum. Dio praises Titus that the Emperor did not use high class women in this spectacle.
Suetonius Dom. 4.1,
Statius Silv. 1.6 and
Cassius Dio 67.8.4 testify that Domitian used women for his shows. Statius referring to only one definite occasion while the other two remain vague about the number of shows.
CIL xiv 5381 & 4616 mentions the duumvir of Ostia being the first one to offer the people of Ostia a gladiatorial show involving women. Since in the inscription the word mulieres instead of femina is used we can assume that they were not of high status.
2.
Fictional Accounts
In
Petronius 45.7 Echion describes that a rather shabby show became a crowd-pleaser because of the appearance of female essedarii.
Juvenal 1.22-23 is about a venatrix named Mevia who has the habit of killing Tuscan boars and holding spears in her right hand with her breast uncovered.
Juvenal 6.246-267 is about Roman matrons practicing wrestling and playing at being gladiators going through their gladiatorial training in full armor and heavy helmet. This is not about them appearing in the arena though.
3.
Legal Measures
The notice in
Athenaeus (4.154a) mentions a man whose will required that the most beautiful female slaves in his household had to fight as Gladiators although in the end this provision was not implemented because the people forbade it as being contrary to law.
The Senatus Consultum of AD 19 from Larinum forbids the appearance on stage of members of the senatorial and equestrian orders and their participation in certain activities concerning gladiatorial combats. This contemplates that women of the upper class might potentially appear in the arena but it does not prove that they actually did and that it was a comman problem.
Hadrian banned the sale of a slave or maidservant to a pimp or a lanista unless the owner gave a reason for doing so (
SHA Hadr. 18.8-9). We can assume that female gladiators came from the same sources as male gladiators, i.e. volunteers but most often by some sort of purchase.
Septimius Severus banned performances by women (
Cassius Dio 75.16.1).
4.
Artistic Evidence
Here we have only one definite depiction, i.e. the famous relief from Halicarnassos which can be found today at the British Museum showing the two female gladiators Amazone and Achillia who fought so bravely that they got a draw (stantes missio).
It is not clear if the funerary relief from Maastricht shows two female gladiators or if the defeated essedarius is just shown in a womanly manner with his knees together and legs turned in to express his inferiority.
For those of you who know Dutch here's the link:
http://www.ljongma.dds.nl/nl/dossiers/gladiats.html
According to Marcus Junkelmann who did a vast research on gladiators the relief shows two male
essedarii, there are no breasts or anything else seen which would imply that they could be women.