If its 50.75 x 1000 and you bid 100, if someone is front running you then you would see 50.75 x 1200 instantly. Yes, there are bots that will add extra liquidity too but if you notice this happens every time then you'll have something to worry about. To really be sure go to an illiquid stock that hardly moves. But I find that bots will generally follow you on the bid, especially if you penny them. I think people probably mistake those bots for their firm front running them.
As for taking liquidity you would look for an offer of 50.77 x 1000 and you take 100 shares and it ends up being 50.77 x 800 with two 100 prints in the T&S.
But does it really make sense for a firm to front run your bids? What if they get filled and you don't? They wouldn't know were to exit. What if they lift an offer and take all the shares there forcing you to bid where the offer is. Maybe you don't get filled. Now the firm has a long and doesn't know where to exit.
Anyways, thats my opinion on the matter. Maybe someone has some first hand experience in front running. Front running would make more sense if theyre doing it to guys holding positions for a longer period. But if they did to to someone who trades a decent amount of shares then it would be very detectable, especially on thin-medium stocks. I doubt they would be trying to front run someone doing only few hundred shares.